Have been on it since beta 3 or something. This has been one of the more smooth transitions, no dev tools broken, and only few apps had some quirks that I filed bug reports for, which are now fixed.
The APFS change really isn't something you are likely to hit, heck you'd have to go out of your way to make it a problem.
Speed ups? Where are you seeing speed ups? I'm on a Macbook (last gen) and have been using High Sierra for the last month or so. Aside from my changed desktop background, I haven't actually spotted (any / many?) changes.
General animation things because of Metal 2. APFS is awesome when copy pasting (instant), Safari's faster, and general performance increase (subjective of course).
Yup, which is pretty neat. Before you'd have to wait for the little Finder window to show up, calculate time remaining and then do the job. Now it's instant when copy-pasting files around :)
It doesn't, it only touches the drive you perform it on (meaning your Mac HD). I literally did a fresh install yesterday with an existing bootcamp partition, and no problems.
One thing though, if you relied on some thing like Paragon or whatever to read your HFS drive, that will obviously not work anymore (not personally a problem, but could be a deal breaker for some).
The APFS change really isn't something you are likely to hit, heck you'd have to go out of your way to make it a problem.
Deffo recommending it :) Speed ups are nice!