I had a TiVO Series 3, which is a close cousin of the XL. Both the TiVO and the 722 are excellent DVRs.
TiVO Pros: Amazon Unbox and Netflix integration, awesome searching abilities
TiVO Cons: Expensive, cable cards can be unreliable sometimes
722 Pros: Fast, can record 3 (722) or 4 (722K) HD channels at the same time with an OTA antenna, 1080p VOD/downloads, very reliable, excellent implementation of external storage as a separate library/archive of content (not just expanding space available for recordings)
722 Cons: User interface is dated (but fast!), Dish's On Demand organization and content is horrible, mediocre searching abilities compared to TiVO