I'm on FF 16 thanks to Nightly. Every once-in-a-while the build will be unstable, but by the next day everything is fine. It should really be called "afternoonly" since that's when I usually get the new build in Eastern time. Periodically I'll get a message that an extension doesn't work. Honestly, the biggest issue for me is that Webex doesn't work so I have to launch that from IE, but I try to do work surfing in IE and personal surfing on my work computer in FF (nightly).

It works so much better than normal FF in 64-bit. On my personal computer (also 64-bit) I use UX since that is actually a 64-bit FF. I had been running normal FF (release channel) on my personal and nightly on my work, but after too many FF issues I switched it to UX and it is much better. I might change my work computer to UX, i'm undecided since I haven't used UX as much and I'm content with Nightly.