I've been using Mailpress for 4 years on my club website. Just upgraded to WP 3.9.1 MP5.4 ... always working with my own newsletter theme.
Yes as some mentioned, the first step to customize and use Mailpress may appear difficult, but don't give up, this is a must have.
Lille French ALpin Club Webmaster
Works like a charm. Highly recommended.
Better than anything similar I've tried. Doesn't have any fancy drag and drop stuff but way more features than any competitor.
Use only if you're not afraid of editing some files on your own! Not easy to figure out since the wiki went down but well worth the trouble if you're willing to invest some time into it!