After market parts

[Little Dicey]

This is my second post on this site. So far I think this is a great site and I have learned many things reading the many threads. With that said I am looking for your opinions about aftermarket parts. In particular, how do they fair? Do parts fit as described or are modifications needed? What have been your issues? Is there one brand that sticks out as being a good one? A bad one? Does one have superb customer service over another? Not trying to beat up on anyone, but trying to figure out what is the best bang for my (and anyone else's) hard earned buck.