Wave is ok, has some great features, but I haven't really settled in to the idea of 'fluid' emailing yet. Having all your emails turned into a series of nested and editable chats feels a bit daunting (I struggle with organisation on a daily basis) but I'm probably just a bit slow on the uptake.
As Kev says, this would probably work really well for group project type things where lots of media needs to be passed back and forth quickly between people and commented on.
For 'regular' emailing type purposes though, the old way seems better.