I ran across these blog posts a while ago and wanted to post them. It's interesting to see how some companies are fairing with MongoDB that have been working with it for some time. No datastore is perfect, but it seems that MongoDB has more than it's share of technical and architectural problems. I know they got $42m in funding recently, but it seems like their going to have to completely re-architect their product to overcome many of their core problems.
MongoDB's claim to fame is a very easy to use API, and it is pretty to start a single node and start developing. That's very commendable. Once you go beyond a single node though, MongoDB gets difficult very quickly. Most other NoSQL Datastores are much, much easier to scale, but suffer on the API side and ease of use. To me, it is going to be much easier to build a better API and put that in from of many of the other Datastores than it's going to be for 10Gen to fix many of the core problems with MongoDB.
Read for yourself and make up your own mind.
Kiip - A Year with MongoDB
ZOPYX - Goodbye MongoDB