ON makes a weight gain that is very low sugar and very low fat. If you use the formula on the canister, you'll drink (?) a mud-like shake that's over 2000 cals. To much. I usually add about 3/4 a scoop to 1.5 cups milk and 2 scoops of ON Pro complex. This bumps up the calorie count to about 800 cals (+/-) and drink 2 a day. Drinking what they suggest is too much in one sitting and will promote fat gains.
Also, the above mentioned protein shake with 1/2 cup raw oats (long cook), 2 tblspns of natural peanut butter and 1/2 a green banana (less sugar than a ripe on) in 1.5 cups of 1% milk is killer and about 800 cals. It's also a great pre-workout shake because of the slower release in carbs for longer energy levels.