I upgraded my iMac, zero problems so far (though admittedly this uses a fusion drive so no AFPS).
As for reasons to upgrade, there's nothing particularly exciting, but things are just a little better. A lot of nice stuff in Safari, though at least most of it is available in Sierra as well, and I'm not actually sure offhand what (if any) requires High Sierra. HIEF support for photos so they take less space. Also better editing of photos in Photos, including the ability to muck about with live photos (e.g. looping them, bouncing them, etc), which is fun. Notes has some good stuff, such as tables, pinned notes, and sharing with other people (which I've made heavy use of lately). And so on.
As for reasons to upgrade, there's nothing particularly exciting, but things are just a little better. A lot of nice stuff in Safari, though at least most of it is available in Sierra as well, and I'm not actually sure offhand what (if any) requires High Sierra. HIEF support for photos so they take less space. Also better editing of photos in Photos, including the ability to muck about with live photos (e.g. looping them, bouncing them, etc), which is fun. Notes has some good stuff, such as tables, pinned notes, and sharing with other people (which I've made heavy use of lately). And so on.