I have installed this product on ceilings of a two family (one up one down) condo. Its amazing stuff. You still need however to get it to really work to try to construct a separate wall or framing system that is independent of the opposite side. This helps prevent additional vibration from moving through the wall structure.
I used a product called Green Glue between two layers of sheetrock. It seems to work pretty well. I"m not an expert by any means, but if you want to reduce sound transfer focus on three things: 1) Mass - the more you've got the harder for the sound energy to get through. 2) Air Sealing - Sound loves an air path - even putting a towel at the bottom of a door will significantly reduce noise coming into a room. 3) Decoupling - Just like @Woodbridge Environmental said. Think layers between quiet area and sound source that are not connected.
Here is the Green Glue website: http://www.greengluecompany.com/
I've used QuietRock successfully. Woodbridge and Ed bring up a good points about being successful with keeping sound out.