The R15 isn't out until November 7th, so near impossible to compare reliably at this time.
The main difference is that no D* DVRs have dual outputs. They can record two channels at once, while you watch a recorded program, but none offer the "two programs on two separate TVs" that the 522 offers. D* DVRs are basically non-buggy 522s in "Single Mode".
D* is offering $100 off -any- DVR (one per household) through 2/26/05.
It's just that the R15 and R10 retail for $99. So they're both free after rebate.
HR10-250s retail price is $599, so you can get it for $499 after rebate.
I had a 522, and ditched it for an R10. I was ecstatic. I've since replaced the R10 with an HR10-250.
H