Interesting one this, I've just been looking for threads on mammoth p after using it for the first time and seeing a 30% yeild increase on both my DOG and Black Triangle clones. Thought the feedback online would be amazing and it is in some places but in others it's dominated by people saying it's too expensive and it's snake oil .
Just to add a bit of perspective, the product has a peer reviewed PhD paper to back up it's claims (unheard of in this industry as far as I can see) and has a dilution rate of 0.18ml per litre (makes it pretty inexpensive doesn't it?) Calling it overpriced is like saying shatter should cost the same as shake.
I listened to an interview with the guy who developed it on the NPK show a while back and he said it was originally intended for large scale agriculture but he found that he would need to licence it to Monsanto or similar to get it out there so moved to the cannabis market as he didn't want to work with them. I can get behind that on a personal level.
Personally I noticed thicker stems and larger harder flowers, I've tried many products over the years and have been largely unimpressed but this one has been excellent.
One thing to note is that your water needs to be clear of chlorine prior to use, I know that might seem obvious but maybe some have not been doing this hence the lack of results. All I can think of really, has worked wonders for me so I don't understand the negativity .
Anyway, use it or don't, makes no difference to me but don't dismiss it based on forum know it alls, try it for yourself.