I buy most of mine from the iTMS. I'm not an audiophile and I end up listening to music while on trains/planes so the quality doesn't bother me.
Pre-iTMS, I bought one or two albums a year; I'm not a big fan of record stores and flipping through rows and rows of CDs. And when I want an album, I tend to want it 'now' rather than waiting for it to arrive in the post. Post-iTMS, I buy one or two a month. I have 1005 songs in my Purchased playlist of which around 200 were free.