At least on OS X, this is faster than Firefox has ever been for me in the last 2-3 months. Switched back to Firefox over Safari.
Although buttons look native-ish, dropdowns (from the <select> element) still look hideous. Check out the following bugs:
https://bugzilla.mozilla.org/show_bug.cgi?id=402625 https://bugzilla.mozilla.org/show_bug.cgi?id=430745
In my experience, Safari 4 has been noticeably faster than FF3.5 RC1. I could understand wanting the extensions and such, but if speed made that much of a difference in your decision, I just can't fathom how Firefox on OS X could be preferred.