I used and loved Mail for years. I still think it’s a solid client, but Apple just hasn’t kept up very well with email trends, and it never bothered to make Mail work properly with Gmail’s admittedly unorthodox features. For example: hit Mail’s Archive button on a Gmail message, and it will create a new /Archive folder instead of moving the message to All Mail.
Sparrow is to Mail what Aperture is to iPhoto, and it’s great. It very quickly became my primary client on Mac, and immediately on iPhone. I’m sure its iPad version won’t disappoint.