We at streak.com use contextIO and have to say its pretty awesome. I'm ecstatic I don't need to know how IMAP works.
Their dev support is top notch - highly recommend.
Also worth mentioning that their API test console is awesome. Anyone making an API should take a look at their console - super easy to test different calls.
Yeah, it's just a great API. Hits an important pain point.
We love streak!
Absolutely loved contextIO when we built our Email Analytics feature at Wishery - great API, and great dev support.