FYI, a BOV is just a vacuum-operated pressure relief valve that goes between the turbo and the throttle body.
When you're at WOT and let off the gas, the throttle slams shut but the turbo is still spinning. All that pressure has to go somewhere. On a stock setup, it gets pushed backwards out through the turbo and air filter.
A BOV has a vacuum line that connects to the manifold on the other side of the throttle body (you can see the line in SyTyJedi's pics). When there's a pressure difference between the BOV and the manifold (i.e. you just let off the gas so there's vacuum inside the manifold, but still pressure from the turbo outside the throttle body), then the BOV opens up and vents that extra pressure out instead of forcing it backwards through the turbo.
This puts less stress on the turbo. However, there are many BOV-less stock turbos that have lasted several hundred thousand miles. I have almost 155k on mine. It's not a life or death issue for your turbo, though it won't hurt.
Realize that a BOV only does anything when you let off the gas. When you vent all that extra pressure out, the turbo doesn't get slowed down by the backwards airflow. That means it will be a little quicker to spool up again when you get back on the gas. This is a much bigger deal in manuals where you're on and off the gas constantly to shift, than in automatics where the throttle is wide open the whole time from 0-100.
A popoff valve is similar, but it just uses a specific spring for a certain pressure, not a vacuum source. It's basically just a pressure relief valve at __psi.
Either one working properly should help your turbo live a little longer and maybe give a slight performance increase in some very limited situations. If they leak, you'll lose boost and performance. If they stick, it's basically the same as not having them - no real effects, other than a false sense of security.
