One of the biggest things in deciding what is good and what is bad is first of all the design of the BOV. This has been discussed before but a lot of the cheap BOV's have the same surface area of the valve face and the surface area that the manifold pressure is pressing on the back side. What happens when you have this situation? A lot of leaky BOV's.
A properly designed BOV will hold an insane amount of pressure because it is designed to always have the manifold pressure holding the BOV closed pressing on a much larger surface area than the face of the valve itself (which is of course also subject to manifold pressure). This creates a huge difference in pressure on the back side of the valve which means it will hold until the cows come home.
Most of the ones that people complain about having to dink around with washers and springs all the time are the first type. The latter type often have adjustments as well but it's really not to help hold it closed, it's to set at what vacuum the BOV will open at.
OK.. that's my BOV ramble for now...
And just to throw it in there.. if you have a drawthrough MAF use a BPV.
