A 6.1L Heads without Catch Can at 36,000mi.
Background: (Another user) Pulled the intake off of a 2007 6.1L with 36000 miles. The engine has never had a catch can installed and as you can see there is a substantial oil build up. (reference the thread Why you need a catch can
A 6.1L Heads with a Catch Can at 62,800mi.
Background: (Mine) I've had a Catch Can on since about 21mi on this engine. I just broke 62,800mi. I was noticing buildup around the gaskets which pointed me to replacing them. In all, I'm a catch can believer.
Intake Manifold Without Catch Can @ 32,000mi
Intake Manifold With Catch Can @ 62,800mi.
Key things to point out is the oil pool at the bottom of the runners on the pics without and the differences between the buildup around the injector ports.
With every oil change, I have collected at least 1-2 oz in the catch can.
Can't speak this from a performance standpoint, but from a pure maintenance standpoint, I'm a believer.