Another advantage for Maildroid
I agree with most of the good things said about Maildroid. I also bought the paid version hours after installing the free one.
I have been running K-9 and Maildroid in parallel for about a month. Both apps look after all my mail accounts (7 in total, a mix of gmail (2), yahoo (1), hotmail (1), webdav (1) and pop (2)). For that one month, I have used Maildroid almost exclusively to read mail (and download attachments and longer messages), K-9 has been just collecting mail in the background. After about a month, K-9 had used about 115MB of data while Maildroid had used about 30MB.
That to me is significant. For the moment, I have unlimited data, but that will end in a couple of years. By then, the lower data usage will be very important to me. Of course, the speed improvement of Maildroid over K-9 is also significant. I get most messages as fast on the phone as I get them on the desktop. It seems the battery usage is also lower for Maildroid, but not very significantly so (4% versus 5%, based on the battery app in my Droid Razr)
One problem: as of now, Maildroid does not seem to support appointments sent from an Exchange account to my Webdav account. Not sure why, I have asked the question from the developper.