I pretty much just watch shows that I get on iTunes because I am too lazy to record and cut commercials, etc. I use to have my PC set up as a DVR, but got tired of it. I only watch Lost, The Office, and Deep Sea Detectives really, and South Park can be watched for free online. At $30 or so for a season pass, that is less than one month cable bill, I could buy season passes to 12 shows and still spend less than for cable/dish. The AppleTV is pretty slick if you want to watch the shows on your plasma. The only drawback to using iTunes for TV is that if you want to have the same show on multiple computers, it can get kinda confusing. You have to activate devices to use them, and you can only have 5 activated. AppleTV is cool because it does not use up a device activation, it just syncs or streams from your computer. The other great thing about iTunes is that when I am overseas, I can still watch my shows, the download takes longer, but it is small enough to still work. If you did Tivo and transfered them to your PC, they would be big. iTunes will automatically format your shows for your video iPod as well.
I know that is kinda a run-on, and may not add much to what's been said already...