I've been using Camstudio for a while, and it's not bad, but I've had some problems with it (probably related to Vista, but I don't know for sure): the only codec that works for me is the MS AVI default, and that means 1) huge files 2) an error message when the file size exceeds 2 GBs (I can still salvage the temporary files though). I've also had problems with capturing audio from the speakers (what Gorfinhofin mentioned); the volume level always resets when I start recording.
The real issue with Camstudio however is that development seems to have stopped (or slowed down significantly), since the last version was released in 2007. Being open source means that anyone could continue from where they left, but there only has been a couple of people branching so far, not going too far.