Sparrow is an attempt to get away from supercomplex, feature-packed e-mail clients in favor of a simpler e-mail experience on iOS. The interface is elegant while remaining very straightforward: e-mails are listed with the newest on top, and all your actions can be completed with simple gestures. Setup only requires that you add accounts; then you can get started right away.
Though the interface is sparse you still get plenty of the common actions and options you'd expect from a basic e-mail app in Sparrow. You can save your draft e-mails; Star (or mark as favorite) specific conversations; add color-coded labels (which you can later sort by); and quickly open and collapse individual e-mails from a thread -- all by using finger swipes. You can also import Facebook pictures for your contacts, directly attach photos and images to your messages, and send quick replies. Sparrow offers support for all IMAP accounts, including Gmail, MobileMe, Yahoo, and custom IMAP accounts, and you can set up multiple accounts and view your messages in a unified inbox.
Sparrow received a couple of new features in the most recent update. You'll now be able to compose messages in landscape mode; navigate between messages by swiping up or down; edit and create labels and folders; and the app offers support for several more languages.
The one thing that Sparrow does not do (and it's a big one) is tell you via push notification that you have new e-mail. Surely, this will be something that comes out in later releases, but for now you'll have to launch the app to see if you've received new mail. This strongly effected my star rating for this app, but I will upgrade it once notifications become available.
Overall, if you're looking for an alternative e-mail client on iOS that's not overloaded with features but keeps some of the most important ones you already use, Sparrow might be perfect for the job.