I think the real killer features of 0.22 are (1) VDPAU, and (2) HDPVR support. Sure, there's other stuff, but those two are the really biggies.
However, VDPAU has also now been available as a solid/stable backport for 0.21 since winter 2009, and is not by itself a compelling reason to risk 0.22.
HDPVR support is also available as a backported patch, but didn't work well in that form when I tried it here for a friend. So that friend has been waiting for 0.22 to try again, though I am trying hard to push him towards SageTV as a better solution.
The current -RC of Mythbuntu will turn final later this week, but anyone considering it might want to let the dust settle for another month of updates after that. I would hope it might be usable by December.