I've been using it since the day after release (I was at work, otherwise I'd have downloaded it that day).
My overall impression is.... Meh, it's okay.
It's basically an IE clone without the memory issues, hacking concerns, etc.
I strongly dislike the new address bar (the fact that it guesses what I want in my address bar, instead of the places I actually *TYPE* in). Sure, I hit the "New Posts" page here on CGA quite often, but it also keeps the main page in my history, taking up 2 spots instead of 1 - subsequently, dropping some of my other "often viewed, but not visited everyday" pages out of the list - which happens to be fixed at 12 entries. tough poo if you have 13 pages you visit daily.
Bookmark handling is quirky, at best. When I want to add a new bookmark, sometimes it shows me all my bookmark folders...and sometimes I just get a choice of between the bookmarks toolbar and unsorted bookmarks. I haven't changed any settings, yet there is no rhyme or reason to it.
The biggest thing I like is that I can have about 25 tabs open and still don't lag out the way I did before. Memory leaks are mostly closed up, too. Huge props to the Mozilla for those issues.
Like Brelan said, get AdBlockPlus. It's a must. Other addons are nice, but this one is just superb. I use EasyList USA as my data source, and rarely have any problems.