These days MongoDB more or less (finally) works as advertised as well. It actually has worthwhile backend options (plural!) and the new (v1 vs v0) replication protocol is actually correct. Eight or so years of development has led to the most egregious of the bugs and corner cases being rounded off.

So, it started as a shitshow on the back-end and people who had to maintain systems based on it hated it, but it got popular because it was easy to get simple systems up and running with. Then, over time, it slowly became less awful as it was substantially rewritten and large portions of it redesigned. I'd say PHP is a very, very good comparison.