They help with a really poorly set up pcv system... so any vehicle built in the last 10 years or so. They work as advertised. You can build your own for a couple of bucks. Off the top of my head, I think @Sparky runs one on his truck.
I have one on my 2006 Pontiac GTO and it catches quite a bit of oil that would have made it into the intake manifold. That being said, I'm not exactly sure about the intake setup on the 360 platform, but I did however modify the pcv setup on my GMC Acadia due to problems of excessive oil getting into the intake tube. It's not my actual video (below), but I did the same modification.
Thinking of it now, I might put a catch can on my Acadia...simply because I'm bored this week.
On a V8, it might be worth it. Not sure if it would on the 4.2. It does have a vacuum line from the intake manifold to the crankcase but the vent hose under the resonator box connected to the valve cover, which is the one that spews the most crud. You can even find a pool of oil in the box sometimes. There's just one thing if you do add a catch can to the valve cover is that the PCM is programmed to compensate for the throttle body getting gunked up. So engineers knew that this was a flaw in the system. Then you have the vacuum line. So is there oil mist going through that one too? It might need two catch cans.