MOCA? Using your cable line to carry Ethernet?

So I'm relating to all the discussions of the problems with wireless networks (flakey) for music, but then if you try ethernet over power lines then you will get noise in your power into your stereo, etc....

Kal R of Stereophile (I believe) dropped a hint that he uses "MOCA".

so I googled around, and from what I surmise, MOCA enabled products use your cable line as an ethernet line.

http://www.amazon.com/Netgear-MCAB1001-Coax-Ethernet-Adapter-Black/dp/B001N85NMI

Am I understanding this correctly?
Any of you tried this?
Can Ethernet (post modem) really co-exist on same line with Comcast (pre-modem)?

Sounds too good to be true.

Art