I have one. I wouldn't say I swear by it, but it is needed to tune properly. It didn't really do anything for me except cause an overboost issue. As long as I keep my foot out of it below 3500 RPM, it's not a problem. As I said a tune will correct it.

I've also got an AVO top mount IC and a Cobb catted downpipe. I'm looking to build a low 13 second DD that is fun to drive.