I really want to love it, I just can't. I've been giving it a chance for 2 months now. there are too many annoying things like:

Language support: It doesn't know which language is it that I write. I have to clarify manually.

I don't see attachment in the window. I have to open it or view it in quick look.

Sometimes it fails at copying an email I just sent into the sent folder.

Postbox include all the contacts from a group, it makes its own list with people from that group. Biggest fail so far.

Even though it worked in the end iCloud support was not obvious and it made me work. I love troubleshooting but that was just annoying.

I really enjoy the search. The conversation interface is great, I find that particular thing makes more sense than the one in mail.

I now use both Postbox and Mail together. The idea of postbox is awesome but something big is missing. Also it feels a little windowsy to me, and that's not a good thing.