Can anyone recommend good third party iOS email clients (especially regarding signature support)?
Sparrow is now available for the iPhone, and it supports IMAP.
The app description does not mention exchange, and it specifically says that there's no POP support, but being that it's such a popular mail client, I figured it's worth mentioning.
Disclaimer: I've never used it either on the Mac or on iOS
Moxier
This is an email client which supports Exchange ActiveSync in the AppStore. But I'm not sure it supports sweet email signatures.