If you're fairly technical and willing to put in the time, there's lots of stuff you can play with. If you're not willing to make your HD DVR a new hobby, Tivo is your only option.
I've been running the same install of SageTV (www.sage.tv
) for 4 years, and even though my version is 2 years old, it has features Tivo may never get. I have plug-ins for commercial detection and skipping, background re-compression of archived shows (@ 10 to 1 reduction), Weather and Forecasts, Context-sensitive IMDB search, CD/DVD rippers, torrent-streamed vids, etc; all with no monthly fee.
Without PC-Compatible CableCard readers, HTPC's are limited to over-the-air HD content, though. If that's not a deal-breaker for you, check out SageTV. If you're a Linux person, Myth is also very good, but hardware support (drivers) can be a problem.
This time I'm taking the easy way out, Tivo HD. The biggest downside to using Tivo with CableCards is that On-Demand and PPV won't work. RCN is upgrading my cable box to a an HD DVR AND renting me 2 CableCards for what I currently pay, so I'm planning to run the RCN box into a different HDMI port for On-Demand.