Are they a good thing to have any downsides thinking of fitting one in the cossie

 

I would say yes they are good, I've got one on my RS turbo and it's much better than the bleed valve I had before. I haven't noticed any downsides since I fitted mine either.

 

I have an apexi AVC-R in mine, awesome piece of kit, it does what i tell it to do and control's it very very well.
Only downside is i can't get a speed signal too it so i can't use the boost mapping function to limit boost to stop wheel spin.