Well there's a few channels that support HDTV nows (one HBO, CNN, Discovery, ect) but there's not really enough of them to warrant the additional price.
I mostly use 480P for movies and gaming. I have and HDTV 40inch screen and the difference between 480I and 480P is very noticable. You see alot of scan lines with older projection based non HDTVs.
However on my PC the difference isn't as BIG since the screen size is smaller (21inch) but the image is a bit smoother and the sharpness is nice and crisp. For the price I paid, I happy with the results. But it's kinda trial and error (if you read reviews about viewsonic's N6 you notice a few people had blue tints on the screen but my monitor didn't have that effect).