Mailroom: a multi-account Gmail client for iPhone
One of Mailroom's best features is the "TrueNew" new message count, which shows both actual new messages and unread messages in a split badge. Multiple account support works smoothly with both Gmail and Google Apps. In terms of UI, Mailroom is built on the Gmail web interface, so you can do anything with it that you can do with Gmail: folders, labels, stars and more. Plus, you get one of Mail.app's best features: swiping a message to delete it.
Mailroom is still new, and it's going to keep getting better. Badging on the home screen is apparently on the way, and that was the one glaring feature it was missing (although push notifications would be nice, while we're dreaming).
Mailroom's strength is also it's weakness: because it's built on Gmail, it feels a lot like a web app. Whether that's good or bad is a matter of taste. If you love Gmail - and a LOT of people do - you'll love Mailroom.