They are great for almost anything, as long as you know the limitations.

For example, most don't have PCI ports, so you can't run a high end GPU, but many of these have decent GPUs that can run most games at 30-60 fps. You may have issues on the most demanding games. It would be a great gaming console like device for a TV.

It isn't going to match high end PCs, but for 90% of people in the world, it would make a more than adequate desktop PC.