Pros= sound
Cons= runs rich everytime you make it blow off, unless you set it up for 100% recirculation, in which case you might as well use your stock BPV.

In short, your MAF (airflow sensor) meters the air and injects fuel for that air, however the BOV (if set up to dump into the atmosphere as they normally are) gets rid of most of that air, which means that there is too much fuel (rich) being injected into the cylinders. You will see no performance gain from running a BOV.