Mini-filters on the side of the valvecover I don't suggest. Breather filters can cause oil to be pushed out of it and leak all over the outside of the valvecover. I was always told that positive crankcase vacuum is good. It helps with ringseal and keeps crankcase pressure to a minimum. Although on turbo and supercharged cars its hard to get a quality vacuum source off the intake side of the turbo or supercharger, every bit helps (better than just a breather). On my supra I don't have a catch can, I just run a direct vacuum line from the valvecover to the intake pipe before my turbo (no catch can in between). I have very little oil in my intake pipe.
I'm interested in this as well, I've seen both the little filters and catch cans. Just got my new supra and it is mostly stock, a catch can would be good to add. But I suck and a writeup + pics would rox0r