Your going to get mixed opinions when it comes to if these are good / bad I think.

The idea is (in simple terms) it simply allows you turn the boost up / down. Alot of people dont like them, and say the can fuck up your car etc..., but it's not really the bleed valve that is the problem, it more the problem that the driver will keep on turning it up and eventually the engine goes bang. However, if they are set correctly and then left alone then I dont see any problem with them.

I have heard of people in the past that show thier cars (eg T28 RST), have sometimes had people go under thier bonnet and play with the bleed valve while the owner isnt looking, so you need to be careful.