Mongo is fine until it's not. It's been fine for us for many months, but once you hit its limitations, it's pretty horrible. We're in this situation right now and we're seriously considering moving back to MySQL or Postgres. Basically, "it doesn't scale" unless you throw tons of machines/shards at it. Once they fix a few of their main issues such as the global write lock and fix many of the bugs, it could become an o…
Have you considered Riak? (I ask mostly because I've been looking at both, having a little MongoDB experience but having heard great things about Riak.)
There are better options than MySQL out there (such as Postgres and Riak probably), but when shit hits the fan in production and you need to quickly bring your servers back up, you'll be happy you chose a tool that has a lot of outstanding consultants and immense amounts of documentation. Finding help for MySQL is very easy. Help for Riak or even Postgres is much more scarse. Also, MySQL is very likely to still be around in a few years. We can't say the same for most of the new NoSQL stuff.
I tried to stay true to this as much as possible in the past and it served me well. I made this mistake with MongoDB, however.