The oil separators are intended to remove additional oil that would normally get recirculated into the air intake. This is part of the emissions but is problematic for a number of different reasons, therefore they are a recommended part by the performance community.
They will keep your intake a little cleaner and on some level prevent detonation from oil vapor occurring, which is a good thing.
Bottom line, put a quality unit on your car, the type that mounts over on the passenger side inner fender tend to run cooler and therefore work a bit better, but they are slightly more difficult to route lines to and from, so there is a tradeoff there.