If your engine is running well, you shouldn't have a large amount of oil in the catch can. Most of the fluids that the can catches are actual water and fuel vapor. Cars that fill the can with mostly oil have other underlying issues that should be looked at. For a healthy running car, the a/o "separators" will do nothing but pollute your oil supply with fuel and water vapor. If its a race application that gets an oil change after every track session, this isn't a big deal. But for cars that run 3k+ between changes, this contamination can become very significant.

Also remember that its always important to check your oil level and make sure you have enough oil no matter what catch can you use. Having a catch can or a/o separator doesn't necessarily mean that the engine can't burn oil. You could still see some consumption, so its important to check it regularly.

Thanks
-- Ed