itunes match scans your library and essentially tags anything it recognizes as purchased, it then uploads anything it doesn't know, it recognized about 95% of my library, even "acquired" files, its $25 a year, and gives you ad-free itunes radio, which isn't bad as a service (itunes radio sucks, but that wasn't why i purchased match)
it also auto adds any cd you put in that it recognizes, so you don't even have to rip it, insert cd, let it add it to match, play on phone, the only downside is that when it plays a song, it downloads it to your phone, so i have to go in and clean up my phone every once in a while