Oh and the primary Con, IMHO (and the reason factory cars never come with a catch can): it's another maintenance task you have to keep up with. If you don't dump out your catch can on time (on our cars, seems like once an oil change would be fine most likely, maybe twice if you're paranoid), the can could eventually fill up too much.
If it filled completely, or even filled far enough that the fluid can creep up the sides to the level of your outlet hose in a high-G corner, you could suck the liquid contents of the can into your intake. The condensed oil vapor + whatever else is thicker and greasier than your normal engine oil.
Imagine pouring 1/4 cup of thick greasy oil straight into your intake in one shot, while the car is at high revs pulling through a corner or whatever. At the very least I'd think you'd get some bad knock for a couple of seconds and have a mess to clean in your intake manifold, but there could be worse consequences. I don't plan on finding out
