I used it about 3 years ago and my first thought was "How broken will multi-document ACID transactions be?"
I still want to like MongoDB, I still miss its style of query vs SQL, but I'd have a hard time advocating its use again...
Sometimes it's tempting to use it for projects that I know will remain small, but even then it's not worth the overhead of standing up a different DB when I have a perfectly good SQL server I can muddle through already.