First of all, it's a beta -- not just a beta, but the first beta -- and in my experience a Linux "beta" tends be equivalent to an alpha release everywhere else. (If that sounds unfair, consider that a Linux beta tends to be improved a lot faster than any other kind of software I've ever tested, but in truth it's not until you hit releases labeled "pre" that you start getting any kind of stability.)
That said, it's really not bad. It crashes every time I try to log out or reboot. KDE settings are missing... well, quite a lot of settings, but there's enough to actually do stuff.
Plasma 5 is mostly nice. I'm a little frustrated with some the changes, and the justification for the changes ("to improve usability") makes me crazy, because honestly "to make it easier to create a touch interface when the time comes" seems to be the much more accurate rationale for a lot of the changes I've seen, which is a perfectly appropriate design decision and a lot less insulting of an explanation.
(Seriously, when I say "why the hell did you do this?" and you say "to improve usability" what you're actually saying is "you don't know how to use your computer right.")
The biggest stumbling block is that a lot of settings have been shuffled around in the kde settings box, so I'm back to clicking on each icon trying to figure out which does what. Learning curve, etc.
But griping aside, it really does look like an overall improvement and refinement to KDE, rather than a sea change. I'm very optimistic it won't be the clusterfuck that moving from KDE3 to KDE4 was.