I've used to it for the following:
1.) to get prerelease listens to my favorite artists. Peter Gabriel - Up was on WinMX about 3 weeks before it was released. The tracks on file sharing also included a very good song that was available on the promo disc, but did not make it to the retail release. Had I not used file sharing, I would have missed out on that track. I justify grabbing the prerelease stuff only because I know I'm going to buy the CD once it's avaialble.
2.) To get copies of performances that take place on TV shows and at radio station appearances. I figure these are really public domain since they went out over the airwaves. Lots of times these are acoustic versions that might not be heard anywhere else. In this category, I also have lots of interviews that many times are streamed on the web, but if someone grabs and encodes them, I can catch them later.
3.) Sample artists that friends refer me to. If I download a few songs and like them, I will buy the album. If I don't like it, it gets deleted off my hard drive. Life is too short to have bad music taking up space!
4.) bootlegs and live shows - these I'd pay for if the labels would bother to release them for my favorite artists. Pearl Jam has the right idea I think in officially releasing an entire tour, show by show.
5.) one hit complilations - OK, here is where I am in blatent violation of copyright. I have 2 or 3 CD's I've created for my wife that have singles she really likes to listen to in the car, but there is no way I would buy those CD's for one song. Fell free to blast me on this, I know it's the kind of things the labels hate, but I'm not going to put this on them with the argument "if music was better, I'd buy it...". Nor will I justify this because I've bought $20k in legitimate over the counter music. It's a very small portion of the file sharing I do, but I know it's wrong. Just being honest here...