Induray:
I have used it and it works. It's by no means a 'miracle' product - it's not gonna magically kill all the sound - but it definitely helps at the lower frequencies.
If you're concerned about performance, check out their website (audioalloy.com) They now have independent, third-party acoustical tests available online, both with and without Green Glue. You can compare that way.
If cost is the issue, it's much better to go with an additional sheet of drywall per side ($8 per sheet, per side) versus going with a lesser amount of drywall and a layer of Green Glue. Green Glue adds $30 per 4X8 sheet, per side to the construction, at the recommended 'dosage'. In most ways, you'll get much better performance with more mass. The downside is it adds thickness to your wall. Sometimes you can't afford to lose the space.
Just make sure you have excellent sealing, regardless of what you decide.
You mentioned in one of your posts you wanted to do some tests and try to find some sort of dB ratio (dB before/dB after) to evaluate performance. Admirable, but in my opinion, this is the wrong way to approach the problem.
Inside a quiet house the 'background' noise level is about 30 dB. If your neighbor comes home and plays his stereo/TV at 90 dB (hopefully the worst case scenario) and you have a standard, wood stud wall (STC 33) separating the two rooms, then his noise will impact you to the tune of 57 dB, approximately. If you're trying to soundproof and reduce his noise to the 'background level' of 30 dB, you need to upgrade the structure to an overall STC of 60. This is not so easy (or cheap) to do.
If you could live with 40 dB of noise (and increase the ambient noise in your space to compensate), you'd need a wall with an STC of 50. This is much more viable.
So, if as you've said, you'd like to do it as cheaply as possible, consult books/websites on soundproofing and find the configuration that achieves STC 50 as cheaply as possible. I can tell you right now, it won't be with Green Glue, Quietrock, MLV, or any of the other 'advanced' products out there. It'll have resilient mounting or structural decoupling and lots of mass.
Personally, what I have found works best is to convert a regular stud wall into a staggered stud, sealed and insulated, with 2 layers of 5/8" drywall per side (or maybe 3 layers on one side, since you don't have access to the other). This achieves an STC of about 57-58, and all the materials I can get at Home Depot/Lowes. And, unlike building a double wall, you keep your fire blocking intact and don't get into trouble with the building inspector.
It really sounds like you're overthinking the problem. I say just go for it - buy a case of Green Glue and do one wall. Stick your ear up to it and if you like what you hear (or don't hear - yuk, yuk..) then do the rest of them. If not, switch to the less expensive way.
Just keep in mind that some of the noise is going to 'flank' around the wall, and you might find you have to do the floor and/or ceiling too.
