My manifold has gone bad twice, and it's the same every time. At low speeds it feels like it's overboosting and it surges a lot. When it's really bad it feels like I don't know how to drive a manual. But it's been the same every time - very smooth when the manifold is good, and it gradually will get worse until it feels like garbage and you get a CEL for it.
First manifold made it to 30K, second one made it to 99K so I'm hoping the next one will see it to at least 150K.