I finally made the jump - I was a way for a bit and I didn't want to have to deal with any issues if something went wrong, particularly with incompatible extensions. The upgrade process was a cakewalk, everything synced up like it was supposed to, and the browser is noticeably faster.
As other have done, I immediately switched my tabs back to where they used to be. Another annoyance is that I was in the habit of double-clicking on an empty place on the tab bar to open a new tab, and in FF4 that actually maximizes the browser. I'll see if I can break the habit.
For those of you who are on the fence because you're not sure if all of your extensions will work, I have found a handy little add-on that tells you whether or not your current extensions will be compatible with a future version of Firefox:
https://addons.mozilla.org/en-US/fir...it-compatible/