I've noticed a trend across about 20+ candidates, all of whom are smart people: people are using Mongo without actually understanding what the hell it's trying to solve by getting away from the RDBMS paradigm.
I'm not sure if this is because 10gen markets it as a general purpose tool, but I have yet to talk with a candidate who can actually describe why they were using the DB vs. a SQL database. I'm all for learning new things, but I can't help but wonder if the string of negative MongoDB posts is coming from people who pick it b/c it's new, then realise pretty far in that this is nothing like a normal DB, and "having no schema" isn't really a reason to go with a tool as foundational as a data store.
I think Mongo is great for really specific problems that its designed to solve. It's probably pretty bad for a general purpose tool, but I'd be surprised if anyone serious actually considers it one.