Tweaked Ubuntu goodness.
Mint = Ubuntu with some initial release kinks untwisted from the get go (Mint stands on the shoulders of a giant standing on the shoulders of a giant...) + a mindblowingly responsive chat/forum community (plus Ubuntu's own, not to mention Debian's) + a few less things to install for my non-programming activities (and those of my family) + I like their sense of aesthetics.
Not necessarily better than the parent Ubuntu but, at the very least, a very likeable and useable *buntu flavor.
(As to what "real developers" use, I have no idea.)
P.S. LMDE (Linux Mint Debian Edition) has been somewhat of a disappointment in the reliability department (although it definitely is snappy). In addition, my suggestion that (vanilla, not LMDE) Mint is more bug free than Ubuntu needs to be tempered: Out of the box, maybe; over time, not sure. In particular, it appears to me that Mint's stability degrades more than Ubuntu's as updates pile up.
P.S.II Mint 11 runs reasonably cleanly so far, and is noticeably snappier than updated Mint 9. However, it appears that the use of an altogether different desktop than the Ubuntu parent (Classic Gnome VS Unity) has led to a surprising number of desktop bugs.