And I'm a fan. Consider some of the differences with iTunes:
Price: iTunes, 79p per track. Play.com, prices start from 65p. Most tracks seem to be 70p.
Quality: iTunes, 192Kbps standard. Play.com, 320Kbps standard.
Format: iTunes, own m4p format, nightmare in terms of compatibility. Play.com, .mp3, works with just about everything (including my CD/mp3 player in the car).
DRM: iTunes, tracks are DRM protected. Play.com, no DRM protection.
I think it will be interesting to see how Apple responds to this.