One of the hardest questions for me to answer is what style of mountain bike I like best.
It's difficult because my preferences sometimes change in a season! At this time I've found myself enjoying a Cross Country (XC) full suspension bike, my Specialized Epic. 100mm of travel front/rear keeps me planted while maintaining efficiency on climbs.
The majority of riding I do is in Decorah, Iowa where we have a wide array of XC style mountain bike trails. A goal of mine is to explore other areas/styles of mtb and in doing so, I'll have to explore other full suspension options to make it more fun/comfortable.
"Why full suspension?"
I've been on multiple styles of bikes with multiple wheel sizes, but the one bike setup that gave me the biggest confidence boost was a XC full suspension bike with 29" wheels. My first XC full suspension was a carbon Salsa Spearfish. Its suspension setup was a different style than the Epic as it was always "active" and I rarely ever shut it off. I got used to how it would compress on uphills and found that it gave me awesome traction on climbs (where some might find it cumbersome.)
I found the whole bike was able to compliment my riding style nicely. Sometimes I'm methodical and I'm riding at a slower pace and deliberately picking lines. When I'm bombing (in my mind) down a hill, I appreciate the give the back end has. It makes me feel more planted on the bike and that allows me to descend faster.
"How did it happen?"
My journey to owning a XC full suspension bike came from entering the Chequamegon 40 bike race. This was an event that several Decorah locals attended and most of them said that a full suspension bike was key to their success and comfort. The course is 40 miles of gravel and fire road- you'll have rocky descents that can be littered with large, rounded rocks. Depending on rain, you might have sizable ruts to contend with. It's not a smooth course, and 100mm of travel can give you just enough plushness to keep your body from totally hating you.
It's a long course, so you want a suspension setup that can be efficient rather than one that is too plush (think 130-150mm+). A fully rigid bike can be lightweight but can beat you up over the course. A hardtail has suspension in the front that can aid in comfort, but over the long haul, you might find your body fatigued.
At the time for bikes to choose from I had a Trek Cali Carbon SLX hardtail and a Trek Lush Carbon full suspension. Neither bike would be ideal for the course, and due to that, we decided to get a full suspension bike that would be a better choice.
"What do you ride?"
My first full suspension was a carbon Salsa Spearfish. That bike opened up my eyes to how fun a XC suspension bike could be compared to my fuller-travel Trek Lush Carbon. It was also my first time being on a full suspension bike where I would have 29" wheels. I found that the combination of wheel size and just enough plushness created my dream ride.
You can ride XC-oriented full suspension bike on pavement and gravel comfortably. When I was riding 30-40+ miles of gravel on the Spearfish, I would lock out the rear shock but left the front open. If it looked like it would be a bit chunky on a downhill, I might open up the rear in order to feel more stable as I descended.
I have yet to ride the Epic out on gravel, but in terms of a setting for the Brain, I would likely set it 2 clicks in from the firmest setting. I don't mind a little give and have used that setting during my last mountain bike race where I was riding a 20-mile course mixed with pavement and gravel.
When I took the Epic down a gnarly trail in Decorah, I made sure to have the front and rear shocks fully open. It worked just fine for that adventure, but going down that trail with a more "trail" oriented bike wouldn't hurt. For the "once in a blue moon" ride down Backbone, it can definitely handle it.
"Why should I get one?"
I won't specifically say you should or shouldn't get a full suspension bike. I think it's a personal decision best made with test rides/demos/rentals. Besides, there are several styles of full suspension bikes out there and you will be the best judge on what will work best for you and where you want to ride.
At this time, the majority of where I ride is XC-oriented, so a bike that has more than a 120mm/100mm setup is likely going to feel heavy and boggy. However, because the goal is to do more traveling in the future and dip my toes into more of a downhill/flow scene with some rocks/drops, I'm starting to make the move on figuring out a setup that would have around 150mm of travel. Why? Because you would blow through shorter travel and that just isn't great for the shock(s) let alone feeling stable and planted.
I think a full suspension bike is a great option to take to other places because it can allow you to feel more confident with riding new/unknown trails. I prefer the feedback I receive riding my Specialized Epic on trails vs. a plus bike or fatbike. I like needling my way through roots/rocks and with a plus bike or fatbike I sometimes felt like I had too much bike to control and maneuver. 29" wheels with 2.3" tires make me feel in control and planted. Having a plush ride feel for XC descents (without the kickback a rigid fatbike or possible bounciness of a plus bike) is more comfortable for me. Time will tell how I like the Evo setup compared to the traditional Epic setup.
Not everyone would say a full suspension bike is the one bike to rule them all; it really depends what you are wanting to get out of your rides. Can a full suspension bike cushion your ride on singletrack? You bet. Can it go on gravels? Yes. Will it be as efficient as a hardtail? Not always. Will you find that to be an issue? Only you can decide.
Do you want to feel planted? Do you want to feel more comfortable over varied terrain without having a weight penalty of larger tires? Do you want a bike that could help you feel more confident on trails you haven't ridden before? Are you looking to feel more comfortable and have less fatigue during mountain bike rides/races? If you said yes to any of those, then looking at a full suspension bike might be a good idea.
There is a lot that goes into figuring out what kind of full suspension bike would best suit your needs.
Where you ride, how climb-intensive it is, and where you want to take the bike.
Not every bike is going to be absolutely perfect for every trail system nor will every full suspension bike be the best for races/longer rides. In the end, you may end up with multiple bikes depending on what you choose to do, or if you don't, choose something that will work the best for the majority of your riding.
"What wheel size?"
29" wheels vs. 27.5" wheels will be something you try out since not all brands will give you the option of one wheel size vs. the other. Between Trek and Specialized, I am only able to get 29" wheels with Specialized brand as Trek has gone to the "smart wheel size" concept by putting on the "fastest wheel that fits." 27.5" wheels are fun, but to make a bike more "multipurpose" for gravel or paved riding, I want the larger wheel size. I enjoy 27.5" wheels on mountain bike trails, but did not enjoy them so much for a long gravel ride.
For how I ride and the versatility I seek, I prefer 29" wheels most times. For me, they feel like powerhouses out on the trails while the 27.5" wheels feel more spritely. For the 150mm travel bike, my plan is to rock 27.5+ wheels so it has a similar standover of a 29" wheeled bike that has more tire footprint for questionable terrain. My thought is I'll be able to roll over obstacles easier and feel more stable when I'm riding down a trail fast. From what I've seen for the downhill riding we'd be exploring, it's fast and flowy, but you might have some rocks to roll over and small booters to "launch" off. It's not so much about technical line-picking (where I prefer smaller tires) as being able to keep yourself rolling with confidence, and the plus size tires sound like a great option for that.
"Is the cost worth it?"
That's a question I feel only you can answer. If riding a full suspension bike vs. the other mountain bike options out there feels best, then yes. However, many do not realize that owning a full suspension bike comes with responsibility. You should get your suspension serviced regularly, not once every 5+ years. If you wait until the fork or rear shock is barely functioning that could mean parts or rebuilding the suspension.
Not all bike shops service suspension in-house, so that means taking the shocks off and sending them in. Either way, servicing suspension costs money and it also means you are without a bike until it's done.
A fully rigid bike does not have that cost.
A hardtail has a front suspension fork which should get serviced, and that costs money.
How much do you value your comfort and overall ride experience? If you find a full suspension bike ticks off those boxes for you, then the cost is worth it. If the thought of maintenance beyond taking your bike in for a tuneup makes you cringe, then maybe it's not the right option at this time.
All in all, full suspension bikes are not inexpensive bikes. Throw carbon vs. aluminum into the picture and that price tag can go high quickly.
Do you need carbon?
It's light and has a way of absorbing bumps rather than aluminum frames. Carbon can create a very comfortable ride, plus I'll mention again that it's a lightweight material.
That means it can make it easier to lift/carry the bike plus you aren't pedaling around as much weight which could equal longer rides.
Folks worry a lot about breaking carbon. I have crashed plenty of times on a carbon bike and have yet to crack a frame. The best part is carbon can be fixed!
Aluminum can be fairly lightweight, but it won't have the same stiffness or compliance as a carbon bike would. One benefit with aluminum is it lets you spend more on the components vs. spending more on a carbon frame and not the components you want because carbon, in general, is more expensive.
You can always add carbon bits, like a seat post or handlebars to reduce weight. Go all out and get a sweet set of carbon wheels!
Getting a bike that can be set up tubeless (using sealant instead of tubes) out of the box is awesome. You reduce weight as well as reduce the likelihood of puncture flats. You can run lower PSI (air pressure) for a more supple/plush ride. However, if you plan to do a lot of gravel or road rides you might want to keep the tubes in so you can run firmer air pressures. Again, it's all about how you plan to use the bike.
As I've mentioned before, the best route to figure out if full suspension is the route you'd like to go is to ride bikes.
There are many bike options out there and only you can really gauge which one would best suit your needs.