I have a few timers that make the guide and timer functions sluggish, and if I try to create or delete timers while all three tuners are busy, the system just reboots. Also of note, my 722
hates SD recordings; the seek and skip functions just don't work as advertised. My hope is that a newer, faster chip keeps the system more stable, too.
Looks like the 612 and TiVo HD are based on the BCM7401 which may or may not be "one better" than the 722k. The clear difference between the 7400 and 7038 is a 50 MHz bump in CPU core speed, but the rest is mostly jibberish to me.