Lately (and not so lately ;) there has been a lot of bad press about MongoDB. We use it extensively as part of our product (on single servers) and it fills this role nicely. It has a few drawbacks, most notably huge disk space requirements (MongoDB has no compression) which we are hoping to solve with TokuMX (haven't tried it yet). It has some other quirks too, but in general it just... works. And I love using a document DB because it lets you use relations just when they are needed. With relational DBs you often have to use complex joins even for things which should be incredibly simple. On the other hand, I do miss JOINs when I need them (though we solved that on app level). And I would appreciate a way to define schema... :) But would I exchange document DB for relational DB for that? No (for the current project).

Maybe I'm getting old, but I really want DB to just work (unlike HBase, at least a few years ago). Other than that, I'm flexible.