Grippy as hell. Avoiding wheelspin means you can pace technical climbs better, whereas on 2.*” you’d peak into the red to keep moving when you lose traction and be in recovery in between.
On looser surfaces, the threshold of grip feels more predictable, so you can hold a fast 2-wheel-drift if you have the nerve. Foot out, dab of oppo, etc.
Downhill, the larger tyre absorbs angled rocks and roots pretty well so doesn’t twitch around as much as a conventional wheel, so you can more often take the line you want, so long as you have the mass and strength to steer against the larger contact patch and larger gyro effect. First thing I upgraded on my fatbike and will upgrade on my GF’s fatbike is brakes, because they descend a lot faster than you think.
In general I think they’re better suited to physiques closer to “track sprinter” rather than “mountain goat”. A fatbike asks more of you and if you’re too weedy you will feel assaulted.
There is a lot of variation in fatbike design that tends to be ignored. Earlier bikes tend to be “retro XC”/bikepacker, whereas fatbikes like the On-One Fatty with slacker headangles and shorter chainstays pander to adrenaline junkies. My GF says her Genesis Caribou (older orange model, steel, bosses everywhere) is quite hard work now that we’re out of mud-plugging season, whilst she says “[my On-One Fatty] just seems to want to go“.