said by jtk2:So is there a specific point in time that this needs to be done, like at a certain mileage or whatever? Or does it just vary from car to car?
No, it depends on what GM engine.. This affected mostly only mid 90s to early 00s. Certain engines like the 3800 (mine) were particularly bad. The ones I took off in the pictures I posted above are from a 98 with only 86k miles and from the condition it looks like its been bad for quite some time. Probably in early stages of a full out failure.
As far as install, if you do a lot of work yourself then the task really isn't that bad, I'd rate it like a 7/10 in difficulty, if of course you set aside an entire day, or maybe go slow over 2 days with a garage (definitely recommend in a garage)... Is it worth $600-1000? I don't think so... Though it did take me about 6~ hours with running into a couple snags, taking breaks and being a perfectionist at cleaning the gasket surfaces.