This has been discussed so many times here, especially recently. Search HN you'll find tons of info.

MongoDB is great at small scale. Basically, if everything fits in memory, it's awesome. If not, it slowly becomes a major pain in the butt, especially because of the global write lock.

I love all the concepts behind Mongo. Replication is so easy, and the data structure is extremely flexible. I just wish the implementation was as good. Hopefully one day...