IMHO, the rankings (all below are external HDD capable):
1. 722K: two sat, no built-in OTA tuner, but with the purchase ($29) of the OTA module to be installed in the box (very ease to do), 2 OTA tuners allowing for up to 4 HD recordings at the same time; larger HDD; PIP; Yes, slight, but noticeable improved PQ. Faster response: better chip? VC-1 decoding for more content; Dual mode capable for independent TV 2 experience; New learning remote with ability to save all your settings--timers, favorites, etc.-- should you have to RMA the box, and then you can download all your settings to the new box. This is powerful and convenient.
2. 722: Two sat and 1 built-in OTA tuner allowing for up to 3 HD recordings at the same time; larger HDD; PIP; VC-1 decoding for more content; Dual mode capable for independent TV 2 experience.
3. 622: This model seems to have problems that the above models don't, perhaps because it was the first ViP STB. However, same recording abilities as the 722; smaller HDD; 2 sat tuner and 1 built-in OTA, but older chip; Dual mode capable for independent TV 2 experience.
4. 612: This is Dish's up to 4 HDTV solution and is comparable to the 722 in most ways (later generation than the 622), but I rate lower because it has no PIP and will not provide a TV2 independent experience. There is an RF Ch. 3 / 4 output that may be connected to a 2nd TV, but it will display exactly the same thing as in the room with the 1st TV. However, it is a 2 sat tuner box with 1 built-in OTA tuner; smaller HDD; VC-1 decoding for more content. Dish will lease up to 4 of these boxes allowing true HD on all 4 of your TV's to which it is connected. Think of the 612 as an HD version of the old 721.
The end.