I used the Gredyy type S, I didnt like it It sounds like a chirping bird, whistling. It functioned well but I got sick of the sound within like a month. Some people have it set very loose so it is chirping all the time, I dont like it.
I have also used the older Blitz super sound BOV, it worked exelent and sounds great, just a sharp air releasing sound, no chirping or whistling. I would have kept it but the valve is quite small and it wouldnt release enough air for my current hp.
I now use the Tial BOV, it has one of the biggest valves available, it sounds allot like the Blitz, just a sharp air releasing sound. It is a little slow reacting because the valve is so big. Sometimes you get a surge just before it opens up. The only way to adjust it is to change the spring, which is a pain, and the springs are like $30 each.