If you've had a love/hate relationship with the included Mail.app on your iPhone or iPod touch, you might want to take a look at Sparrow ($2.99 in the App Store) - provided you use a Gmail or Google Apps email address.
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I can't do a proper review because I've already spent too much on apps recently. Maybe next month?
The main reason I see people not liking Sparrow for iPhone is that they don't include Push notifications1. Personally it's not that big a deal to me because I prefer to check email when I want to check email, not when my Mail app decides to check email. And the work I do isn't ever that mission critical that an email would make or break something serious - especially when all my clients have my phone number if they really need to get in touch with me.
I also respect the reason Sparrow's creators give for not having push notifications now. They don't want to have to store your username and password:
This is a responsibility we're not ready to take. As a startup focused on iOS/OS X development, we do not have the skills to secure your data on our servers and we do not want to put sensitive information at risk. That's why Sparrow iPhone 1.0 doesn't do push.
They tried to do it a different way but Apple wouldn't allow it so for now there's no push.