I have used the gravity, purple maxx, and the soon to be in stores Snow Storm. Gravity is a nice product that doesnt really create bud swell as much as it does help dense up your buds. If used how the bottle suggests, your yield will increase due to being more dense but if used too early in the flowering stage your buds will be TOO hard and almost rockbud like. Purple Maxx is a nice product that does definatly increase resin production but unless you strain has back genetics of blue/purple in it, it doesnt really turn your shit purple. Snow Storm is another crystal enhancer that i am using currently and will let you know the result...so far looking promising. Both gravity and purple maxx have high ppm so when given to your plants, I recommend doing it on the water days not the feeding days. Hope this helps ya.