With MongoDB and lack of hard schema requirement doesn't mean your data model can be all willy-nilly. You have to put some thought into it. Several people have mentioned they are looking for the benefits and drawbacks. I'm really enjoying it. Here's my short list draw backs.
Case Sensitive: If you load data with mixed case you will have to use a regular expression to find it all.
Data Type UnStrickness: If you load zip codes as a string "11223" and try to find it with an numeric 11223 you are out of luck.
Also, if you load your data with mixed data types for the same set of keys, good luck finding it again with out having to work for it.
Rich Documents Are Cool: Until you try to remove a struct inside and array that's inside a struct. There is little documentation that explains how to manipulate complex data structures. You can join google groups mongodb-user, they are very responsive. Plus learning to use JSON as your query language is a different way of thinking from Sql Queries.
Things I like about mongodb:
It is fast.
Replicating Shards is very handy and relatively easy to do.
It has a growing audience and user base.
And finally it fun to watch peoples faces when you tell them "I replicated a bunch of shards today, what did you do?'