The UI of the Gmail app, especially on an iPad, looks a lot like the new web UI that Google launched for Gmail yesterday, which makes it easy to find your way around. The Gmail app also offers all the standard features that you'd expect like searching your entire inbox, push notifications, autocompletion of e-mail addresses, and organizational tools with full use of labels, stars, archive, delete and spam reporting.
Google has even added in some gestures to make navigation a little smoother. You can pull down on your inbox to refresh, and swipe to the right to get a list of all your labels without switching away from your inbox. Overall, it looks like a pretty nice app, and it definitely offers a solid amount of Google options for those looking for more control compared to the standard iOS Mail app.
Gmail for iOS requires iOS 4+ and is available for free in the iTunes App Store.