I set it up so that it is not attached to a cable box. I split the data line coming into my modem and put one directly into the slingbox. The other is connected to the modem. This way he can watch any standard cable channel and won't affect our TV.
As far as I know you get one user per sling box but you can have multiple slingboxes set up if you want. I did read something about a guy who has played with a PROXY and duplicated it to get two viewers but they have to be watching the same channel.
The set up is pretty flexible. It can control more than one device as there are 3 video inputs, Coaxial, S-video and Component. There is a IR transmitter cable that you can place in front of the IR port on the device you want to control. The Slingbox will then act as the remote based on what the user does on the PC.
My biggest question is the picture quality. Does it get grainy or choppy.