I have a 500GB in mine. I have about 200GB of video on there. Probably another 100GB-200GB between Demos and Game Data. I play a lot of games though.
I also have a 500GB external connected to the USB port with all video.
The one thing I hate about my PS3 is the menus for video on the internal drive . If you have a lot of movies, it's a pain to navigate. It would be nice if they allowed creation of subfolders on there.