SageTV is another great option. It comes in server/client configurations is you want.
They have a compatible client for all flavors, Windows, Mac, and Linux. There's a Placeshifter client for shifting TV across the web, ala Slingbox, and an inexpensive extender (the SageTV HD Theater) if you don't want to build your own client.
Out of the box hardware support is unmatched. I currently record from cable boxes with three Hauppauge HD-PVR's quickly changing channels via firewire. Out of box the software user experience is good, but the developer community is thriving and plugins make the experience one of the best.
- Batch Metadata Importer (automatically downloads FanArt;poster, banner, dvd covers, etc.)
- SageMC Custom Interface (a massively customizable user interface with options up the wazzu)
- SageMovieWall (a set of fancy views into your movie collection)
- TVExplorer (combines recordings and downloaded videos into one easy to use view)
- Netflix for SageTV
- Pandora for SageTV
The list goes on and on. Getting the UI exactly as you like isn't for your average computer user. But then again, you did ask this on SuperUser.com. :-)
In addition, SageTV handles music, DVD's, photos, pretty much any media you can throw at it.
Besides Windows Media Center another choice is BeyondTV. BTV is great for the average computer user looking for a simple out of box, TV only, PVR experience.
I've tried others over the years but always come back to SageTV. It'll be interesting to get my hands on Windows 7 Media Center...