I've been running OS 4 beta 1 for the past few days, and for the most part, I've been happy. I love the "Folders" feature, multitasking is great, and the universal inbox is awesome (given the number of e-mails I receive, it was a welcome addition). That being said, it has been a bit buggier than I would have liked. Between my music randomly being placed on shuffle mode (no, iPhone - I wanted to listen to Lionel Richie, not Paula Abdul), occasional e-mail problems, random reboots, and the quick battery drain, it's enough to be frustrating after a few days of experiencing it.
Apple today made OS 4 beta 2 available to developers, and while it's still a beta (and a young one at that), I'm hoping that some of the kinks are worked out. I'll be sure to keep you updated - in the meantime, let us know if you're running OS 4 beta 2!