I like my 721 alot. I can't have a 522 cause charlie doesn't like my current cash flow, he only likes new(leased) cash flow...
But I guess it really comes down to taste and preference. I saw a 522 at Radio Shack, it seemed slow to respond. Not that the 721 is a cheetah. The 721 has no DVR fee no matter what package you have.
The only difference is that the 522 has 2 tv outputs. If you don't need the extra output, then go for the 721. I have a 721 and it works great (so far).
If I went with a 522 I would likely de-activate one of my standard receivers and leave it in duel mode most of the time to feed 3 TV's. If I got a 721, since their is no DVR fee I would likely add the 721 to my account and leave both my standard receivers active to feed all 3 of my TV's. If my initial out of pocket $ is the same, I would choose the 721.
The crappy thing about the 522 is it's a lease only receiver. So you'll never own that piece of equipment. For some, that may be preferable, but not for me. Anyone got a good 721 for sale cheap? LMK.