how good are burnt dvd's?

I have a couple buddys of mine that are burning a bunch of dvd's for there own collection. They are always asking me if i want any or want to borrow them. My question is are they just as good as store bought ones or even dvd's I get from Netflix? Is the audio quality the same? video quality? Is it noticable? If its not recorded the right way, could it hurt my system? Speakers etc? I would just hate to spend all my time and money trying to build the perfect system, only to cheap out in the end. thanks for your help.