I just downloaded it yesterday. So far - I love it!
First and foremost:
It's faster, noticeably. Big improvement!
I like the cleaned-up and improved interface.
It's cleaned up nicely, yet still familiar enough to not have to re-learn.
However I still do prefer the tabs BELOW the address bar, so I'm glad there's the option for that.
My only complaint is when you set the tabs below the address bar (and you're using the Firefox button with no Menu Bar) then the title bar is left blank - without even putting the normal page title stuff in there.
Fortunately, there's a add-on that does this, but this should be a standard feature.
Overall, I like it a lot.
Good improvement, and a worthwhile upgrade!