For the last 6 months I've been getting by with Dropbox's Mailbox. But it's a bit of a stinker really. Search is appallingly limited (about a week, at most). So many things don't work properly. To find something I always hit command-F which in every other app I've ever used is Search, but in Mailbox is 'forward'. Eurgh.
But I like the lists, and the way it lets me empty the inbox quickly. I also like the font and text size, I find Mail somewhat old fashioned by comparison.
But I;m not a power user, btw.