There are many threads on this if you search for BOV. Basically, the stock valve is a recirulating valve that vents air back in to the intake track. When you use an aftermarket BOV, it vents the air to atmosphere. The air has already been metered by the MAF sensor so what ends up happening with an atmospheric BOV is that you get a slight rich condition during shifting. It's not really a big deal. For most people, sticking with the stock valve is fine as it holds pressure pretty well. However, it has been known to leak so some choose to replace it with an aftermarket BOV or crush the top in to increase it's holding capacity.