I had wanted to build a HTPC since the 90s, before DVRs ever came out because I thought it would be cool to record TV and watch it later. I actually bought a TV tuner at one point, but my hardware wasn't really up to it at the time .
I finally built a real HTPC about 3 years ago. I was able to convince my wife we wouldn't need dish any more, saving ~$65 a month and I got a huge unexpected OT check. The HTPC cost about $650 with 2 tuner cards and a blu-ray drive ($130 at the time).
I love the DVR, and the quality of OTA material. I also love that I can do anything with the files I want. I also use Comskip, so I almost never even notice commercials. My skip forward has several skip lengths so I don't have to hit skip 80 times, like during the Olympics. The functionality for movies is also a little nicer than a stand alone player.
I also use the HTPC to play all my music and we use it for Karaoke, which is really fun. Of course I also stream shows, especially ESPN3. Sometimes I use the internet, watch online videos (break.com, etc) and Pandora. I also enjoy playing old video games on it. It is also fun to play flight simulator on the big screen.
Since my other computers are laptops, I also use it to back all of my personal files.
After having the HTPC for 3 years with no cable, when I travel I think "Hey, I can watch cable!" Then I get into the hotel room and it drives me crazy, there is nothing on and what I do watch is full of commercials.