I am replacing all the sheet rock in my entire house. The insulation behind it all is garbage. A friend of mine told me to use cement board for all the walls and ceilings. I am trying to find out if would be a good idea or not. It sounds good for sounds proofing between rooms and all but I don't know of disadvantages in doing it.

I am not clear why you are doing all of this work, is the sheet rock damaged or moldy? Your friend doesn't sound like he does much of this kind of work. What do you want for your finished surface of the walls and ceiling when you are done? Soundproofing is best done by increasing the insulation and installing a deading channel under the drywall.

Disadvantages -
- a pain to cut
- really heavy
- terrible finish
- expensive.

Cement board is really meant for areas where you will tile over top of it. It would be a mistake to use it anywhere else.

What happened to the insulation behind the old drywall?

WOW, Installing cement board in your entire house is so bad. The added weight alone will cause you plenty of problems. I also would ask why you are gutting the house. If it is interior sound issues you will get further using a "sound batt Insulation". There is absolutley NO good reason for installing cement board in your entire house... I still can't grasp the fact that some one acctually sugested this.
Mike