I use Aquamail.
It's pretty damn good, and the author readily takes suggestions on the website on how to improve it.
Far more customizable options than most email clients.
Let's face it -- there isn't a good stock email client for Android. Perhaps I am saying that because I came from iOS and see how good their email client is.
I am *hoping* now that Google bought Sparrow, that email client becomes available for Android in the near future.
Until then, definitely recommend Aqua Mail.