Correct, it condenses the oil vapour and sends clean(er) air out the other side, which can be vented to open air (technically illegal I believe) or into the 'hockeypuck' PCV valve.
How much it catches and how hard it is to drain is entirely up to which one you buy... If you want it to catch most of the oil, get one which is internally baffled at least, and preferably has some form of filtration medium inside. These will be more expensive, but will be better designed products. Saikou Michi ones are pretty well-known, good quality catch cans, I believe.