I have all the older line of humbolt, I just picked them up a few months ago but wasnt too keen on throwing it in the mix on my last grow because I was too close to the end and didnt want to play with it. From what I read its just as was said above the stuff works as advertised, I've watched a friend document the products and the stuff definitely works. He grew some nugs under cfl's and flouros that you would think had to be under HID's.
The purple maxx is basically the same as snow storm ultra if I'm not mistaken its just you use the purple maxx with colored strains particularly purple ones and it helps bring the color out as well as the same thing the SSU does for resin/trichome production I believe. But in order for purple maxx to work color has to be a genetic trait and I would also say make sure you know for a fact that your particular pheno has purple in the genes by growing it previously and knowing it has purps in it. I seen some pics of some insanely purple querkle that P-maxx where used.
As I said I have yet to use it but I will be doing so as soon as the time comes next time around. Just be careful dosing it, go lower than the recommended doses until you get familiar with it. From what I've read here and else where thats the key, getting the dosing and timing right with your particular strains is when you'll see those unreal results. When you apply the dose of gravity theres a massive uptake of nutes, so I am thinking making it the last full dose of anything other than a molasses/carb product and straight water until the finish line is crossed after the gravity.
The one thing other than lower your PPM's when feeding is leave yourself plenty of flush time, the stuff works but definitely flush properly if not excessively.