I just installed it and was able to use it for about an hour before falling back to 3.x.

Strangeness:

* periodically lose the ability to use the mouse wheel to scroll,

* scroll bar would jump to left side of browser,

* forms would no longer submit (and I get a message telling me that I am submitting a form across unencrypted sites; say 'continue' and it doesn't),

* 'options' dialog window blank except for 'ok' and 'cancel' buttons,

* add-ons I use on a daily basis not updated yet.

I'll wait for 3.5.1 I guess.