I've been running the beta for a couple of months now, testing sites I've been building on it as I go along. I can tentatively say that yes, it falls into the "all the rest" category for most common css/js issues that have plagued all its predecessors to varying degrees.
It has still felt too slow, bloated and plagued with dialogues ("I can run faster, disable add-ons", "This page wants to use flash" etc) to serve as my main browser (currently FF for work and chrome for play), but the sooner IE users move to IE9 the better for front-end devs the world over. Thumbs up from me to MS for IE9.