A nice thing about them is the size, if you get one with AGP you can make it in to a very nice media pc, thats what i plan to do when i get a home of my own. is make is a media player inted of going out and buying a dvd player..
Like what every one else has said there is very little room to upgrade. If you slap a good video card and a lots of storage you can make i good media pc. As for using it as an every day desktop i would say no, they tend to run hot if you don't have alot of fans.