I don't use it for computer storage either - I have a 3TB backup drive and a 2TB media drive connected to my desktop for that. 500GB would get me nowhere.
Looking at the contents of the drive right now, I have about 150 feature-length movies on it and probably at least 50 seasons of various TV shows. (Edit: The TV show estimate is probably low because just my Star Trek collection comes to like, 28 seasons, lol.) For me, the whole purpose of the drive is to have access to an extended collection of video content when I don't have an internet connection capable of watching Netflix/Hulu/
Amazon.
Important thing to note: Video files must be in a format that iOS can play natively. This means no avi files, no mkv files. MP4 or M4V work the best. You can work around this but it requires fiddling with extra apps, and it's easier, IMO, to just use something like Handbrake to convert the files on your computer.
I've never used it to back up photos from an iOS device, but I just tried it and apparently you can, but it seems to be a one-by-one process, which seems unintuitive and kind of annoying. Maybe I'm doing it wrong, haha. You won't be able to back up apps on it, but I think I'm parsing your last sentence incorrectly anyway.