I got a beta invite and have played the Barbarian so far. When they released the latest patch it was laggy for about a half hour, but cleared pretty quickly. Must say that while the beta doesn't offer too much, it does certainly show how much more improved it is over D2.
I've found a few bugs, like the HUD saying I have an empty equipment slot, but I don't. Other than that, it's smoothed out quite nicely. I can't wait for retail release!
Yeah that one bug has been posted quite a bit (HUD issue) over on the Beta Forums.
There are also achievment issues that haven't been sorted out yet either.
But, its getting there for sure.