I've been using it for a few weeks now (the betas and RCs). It's solid and integrates better and while it is pretty fast, in my experience, Safari still holds the crown as faster browser.
I do notice though the increased responsiveness of the application itself. Firefox used to be too slow (the actual application, not the rendering engine) for it's own good. It's nice to see that it's faster.
EDIT: Just tried the HTML5 video capabilities (can't believe I didn't try this earlier). It used quite a bit of CPU power and the sound lagged. Now, it could have just been the video itself. I'll see if it happens with other videos.
EDIT #2: It wasn't just that video. The second video I watched (
here) was jittery at the beginning and while the sound was synced, I was still getting high CPU usage. I was hoping that this would have been the alternative to Flash's unnecessary high usage of CPU resources...
