A good idea
An oil catch can is a good idea, especially for diesel engines. They "catch" the oil from the crankcase breather that gets fed back into the air intake after the air filter. Not the best thing for the engine. Removing this oil vapour helps keep clean your inter cooler, and reduces the amount of sticky black gunck that is created. This gunk is created by diesel particulates from the exhaust gas reticulation (egr) valve and the mist from the crankcase mixing if the oil isn't removed.
I was curious how much oil had been removed in the last 15,000 km traveled. This meant getting to the drain valve on the catch can. The drain valve is at the front of my vehicle (I installed it so very easy access). The procedure equates to open the valve, drain the oil into a container, wait, close the valve.
The container I had was a left over drink bottle, and below is how much oil the catch can had stopped from going back into the intake.
A close up picture of the bottle better showing how much oil I managed to extract, estimate about a third of the 500 ml bottle.
Seems to be about 150 ml for the 15,000 km traveled since the last drain. Not a bad effort, and glad it isn't going back into the engine.