Gorgeous game, plenty of personality, fun puzzles and abilities. If you have any appreciation of the action/platformer genre left, I highly suggest checking this out. I know I'm going to be picking it up. And I'm definitely curious to see how it plays Co-Op, but it was perfectly fine single player, at least in the one demo level we got.
If I have a real complaint, it's the Thief character. It takes about two seconds to "charge" her bow to minimize arrow drop over distance, and while it helps, the arrow still drops a considerable amount over the one screen-length... long enough to occasionally make aiming annoying. If you don't charge the bow, the arrow just sorta flops out. And it didn't seem like it packed much punch. So you have a weapon that has a slow firing rate and is not particularly devastating. The end result was that it was almost always preferable, in single player, to use the Knight and just advance with shield up to get in range of archer-enemies, rather than try to take them out from range. It may be that the Thief really only comes into her own in Co-Op when the Knight will be there to provide cover, we'll see. Or maybe when she levels up some more. Or maybe I just suck with the Thief, but I found Billy McBoxmaker, the Wizard, to be easier to use in combat than the Thief.
As far as performance, I run at 1680x1050, I set the detail to "ultra high" or whatever on the launcher, and it ran without a hitch on my 2.4 ghz dual core, 8800GT 512, 4 gigs ram, running Windows 7 RC, so no complaints here.