If you are talking about the General Hydroponics Maxi-Grow powder then yes I have used them. I find that it disolves easier in warm water. I think I usually used between 1/4 to 1/2 tsp per gallon of water for lettuce.
I have lately switched to the Flora-Magic since it is a more general purpose nutrient so that I don't need to buy separate grow and bloom formulas and I can therefore get a discount for buying in bulk.

If all I was growing was lettuce, I would probably still be buying the Maxi-Grow. The one 1.5 lb package I had of it probably lasted me 8 months.