I like the discovery/enrollment process.
Not a big fan of the UI itself, or performance after sign up, at least as it is right now. The only element which seems visually distinct to me is the far left menubar; everything else kind of blends together in a too-widely-spaced collection of random boxes, not even lists. I like native OS widgets. This isn't a video game, don't make me figure out some new convention just to scroll a list.
Search is nice (which is the only reason I still use anything on gmail). Inspires me to try to get lucerne working on my own mail again.
Also not really a fan of giving my login credentials to a cloud service for no particular reason. There's no reason I shouldn't be able to keep them within a client only, and just go through the setup each time.
Realistically I'm unlikely to move off mutt (for intensive email use) or gmail (for infrequent use from other machines), but I'll try it again someday.