First impressions are very good. It's extremely smooth and responsive, the experience when replying to an email is excellent, and it feels an awful lot lighter than Mail. The people paradigm in the sidebar didn't take a lot of getting used to though it'll take a fair bit more use before I can decide whether I prefer it. At the moment it feels to me like there's another sidebar missing, one that would take me directly into conversations.

Only drawbacks I've found so far: search seems to make it go a bit sluggish and it's not that easy to actually find what you're after. I seem to have sent it into some kind of spin by hitting the sync button which brought up this error:

An error occured while syncing account [redacted] (2001):

Could not parse command

Overall though, I'm liking it and I'll give it a go as my primary email client for a while to see if it can replace Mail for me.