I like Canary Mail. I really do! I found it while looking around for better email clients for my iPad and Android tablets, and I really like the look and speed of Canary Mail. I also like that it connects me directly to my email servers, rather than trying to filter everything through their own servers first. So, why three stars?
Canary say that security is one of their priorities, and this is why they don't glom on to your email the way so many other programs do these days. However, there's no way to encrypt your email in this app. Not even SMIME, which is supported by the iOS mail apps, let alone GPG.
Secondly, no rules. I get hundreds of emails in a week (hundreds of spam messages a day) and I need to be able to filter them into their own folders. Apple's client does this rather well. Canary -- no option.
I realize the app is free. I like free. I think it's beautiful, and it's fun to use, but without smart folders, rules, encryption, and so on, it's only half an email program. I wish K-9 Mail made an app for Mac, and I might even get desperate enough to try to compile it!
If your email needs are simple, and you're a member of the new media "share with no boundaries or fences" crowd, you owe it to yourself to try Canary Mail. If you liked it when letters had envelopes, or you need to organize large amounts of email, you need to keep looking, or learn to love Apple's really quite functional mail client again.