Radio 1's Live Lounge is a great collection of really good music. It's probably worth it just for the Arctic Monkeys covering Girls Aloud. However, the rest of disk 1 is excellent too. Plenty of covers and acoustics from lots of very talented artists.
I don't rate disk 2 nearly as well, although that's mostly because I don't really like the artists on it. Some of them seem a bit self-indulgent, or otherwise potentially a bit 'one trick pony' to me, but that's just me.
All in all, if you're after a good collection of really well produced and performed music, then look no further.
Get it at HMV