I used Green Glue and am happy with the results. I have a HT in the corner of my basement where one wall faces a bedroom and the other interior wall faces a hobby room that is overrun by Simpsons memorabilia. But I digress.
I don't know what the results would have been going with single sheetrock (didn't do an A-B comparo), but I can listen to a rock CD really loud in the HT and then step outside it and notice a huge dropoff, like it's just minimal background noise. I know this is very vague - I probably need to get an SPL meter to quantify this, and help with calibration, etc - but, again, I'm happy.
Note: I highly recommend the use of an exterior door, or other such solid door for the entrance to your home theater. If you use a typical interior door, it's hollow and will sabotage your soundproofing efforts.
