I've been using the free version of Sparrow on the Mac for a few months now. Why? Well, I was turned on to Postbox, a pay-for-a-free-email-client-based-on-Thunderbird by a friend of mine at Wayne State University a few years back. I liked it quite a bit because it offered features that weren't part of Thunderbird and didn't seem to be part of their roadmap. Since then, Thunderbird added the whole unified inbox, as the queues were taken from smartphones with their unified inboxes. Thank you RIM. Yes, RIM, not Apple nor Google)!
Well, Sparrow takes the whole e-mail client that should be free (those that come with your OS or Thunderbird) and makes it a bit like Twitter. Quick and to-the-point. No frills. It's very slick.
Now, Sparrow is on the verge (no pun intended) on releasing (if approved) an iOS app to make e-mail much simpler. Dom Leca of Sparrow talks about it here, in this article. He mentions some of his ideas and also talks about what Google did wrong with their GMail app. Me, I find the GMail app pretty weak. It's nothing more than a wrapper to a browser-based GMail client. I never thought it was very sophisticated nor did it gave me any reason to use it over the native mail client on my iPhone 4S. Also, if the search in the mail client is adequate, I never use the GMail app. I simply go to GMail in Safari or Atomic.