Mail.app for iPhone and iPod touch sucks. Sparrow is a flawed gem, but still much better. Worth using.
Sparrow only works with iMap, doesn't use Push notification (Apparently Apple declined to allow it), doesn't have a native iPad version and suffers from the normal iOS foibles like not being able to access any files except the Camera Roll, and can't be set as the default mail client.
That said it's really lovely to use, works with any IMAP based mail service, integrates tightly with Google and does almost everything that mail.app should do, including letting you swipe mail to reveal common tasks (Like reply, or send to a folder), an easy way to flip between inbox, unread, and Gmail's 'Priority' inbox if you enabled it, user avatars to show you who's sent you mail, a unified inbox, and a rather stylish way of marking your Gmail based emails with colours that match the 'label' (AKA folder) that they've been assigned to.
It also handles threaded conversations and simple batch operations like "mark all as read" or "Move" and so on.
At there moment the lack of Push seems a major obstacle, but it's set up to let Mail.app handle all the push notifications and Notification Center things, and then you can just open Sparrow to actually deal with your mail
The killer App
Sparrow allows you to add Aliases to your accounts so you can send your e-mail form a different address to the one you receive it at, or if you need to, switch to sending from any other accounts you added to Sparrow.
It's tight integration with Gmail means that you get all the benefits of your Gmail account - storage, address list, user icons, folders, colour tagging, threaded conversations and snappy IMAP integration.
You can also configure your accounts to only sync specific folders, define Signatures and tweak Sparrow to get a nice experience.
The user experience is what makes Sparrow worth using, despite it's flaws. It's flaws do not in any way outweigh it's good features, and over time it will improve.