The Verge has posted a detailed review of Sparrow for iPhone.
The app is not available for download right now but should be here in a couple of hours/days.
Sparrow is already available as a mail client on Mac OS X and the iPhone edition has been long awaited.
The app would provide a solid alternative to the Mail app on the iPhone. Google recently launched their official Gmail app for the iOS platform. Sparrow provides an alternative to it too.
The app supports Gmail, Google Apps, Hotmail, iCloud, AOL, and usual IMAP accounts. POP support is expected to be added in a future update.