The biggest advantage with these is their size. The bigest disadvantage with these is their size. Confused yet? Well the thing is, the size makes them easy to fit into small locations, and easy to move should you need to move them. But the problem is they have 1 AGP slot and 1 PCI slot and the rest is built onto the motherboard. If you want a quality sound card or a capture card or a better ethernet card then you wont have enough room.
If you really want a small computer, get a laptop. Otherwise get a mini-ATX or ATX tower PC.