I love Napster. It's 9.99 a month for unlimited downloading and streaming. Seeing as they add like 20-30 chart albums and singles every Monday, you definitely get your moneys worth. They also have a massive back catalogue.
I actually have the 14.99 package, which means I can load the music on to 2 compatable music devices, which is awesome really. You could leave the house with a GB of different music every day (altho I've been late to work on a few occasions when the downloads were going slow!)
You can install Napster on 3 different PCs on one account as well, which is great. I have my main PC, my HTPC and my work machine. Your 'library' works across all 3 machines.
The only downside, is yes, all the music DRMed up, and you can't burn CDs without it cost extra. Also, you never 'own' the music, but that doesn't bother me. I pay Sky like £50 a month for TV I never 'own' either
Napster is the best way to download/experience *a lot* of music for a flat fee.