My 721 is quite reliable. And it's five predecessors were all quite reliable, right up until they failed.
I also have a 508 & a ViP622.
Frankly, for avoiding future hassle, I'd move to the ViP622 now, rather than have a non MPEG-4 DVR that will need to be replaced in another year or whenever you get an HDTV. You might look around, you may find an HDTV for less than you think.
BUT- if the 721s aren't broken yet, I'd stick it out until they are. It might be worth your while to get the DHPP. Then when one fails, they will replace it. And I know first hand, they have some on hand.