I have used Bush Master and Gravity, and now going to try Purple Maxx.
Bush Master stunts growth and pulls nodes in, basically throws them into flower. I use 1/4 of the recommended dose and just run it for one day in between Veg/Flower. My local hydro guy recommended that and it works great! If your buds are kind of spread out, or you just want to shorten the flower period a bit this is a good product.
It is a kelp based product and is basically poisining the plant, so use very sparingly. Like I said, 1/4 of recommended dose for just one day is a lot more gentle.
Gravity totally intensifies the density of the bud...takes down the fluff factor...
FYI - I use the Canna product for nutirents because it is perfect PH for me and Sweet Leaf for carbo boost.
I'm going to try purple maxx to see if it helps increase oil/resin...
Check out these:
Humbolt's Own product test grow - Bushmaster, Purple Maxx, Gravity - WeedBay Forums
BushMaster Information Thread - The Garden's Cure