[Lord Nikon]

First and foremost yes they are illegal to connect to the NTL or Telewest Cable infrastructure, to use one as such is theft of services.

However, since it is not illegal to buy, sell, or own one, and they have other LEGAL uses as well, I'll explain a little about them.

It's effectively a remote controlled linux based media center system. It can play media files such as movies etc over a network from the other machines you have, you can install stuff like VNC on it to control PCs etc, it's a networked MP3 player.

Being linux based it's incredibly stable, and you can do a lot with it if you know linux.