I used Mega Mass 2000/Giant Mega Mass 4000 for several years, when I was in college.
It's very carb-heavy. Five ounces of powder gives about 105 grams of carbs, 27 grams of protein and a gram or two of fat (yielding around 550 calories).
The way I used it was I mixed 10 oz. of powder with water: 1100 calories, 210 grams carbs, 55 grams of protein, and 2-3 grams of fat. I took that three times per day, the last one consumed 30-45 minutes after training.
My calorie consumption was between 5000 and 6000 calories (though, I've gone as high as 6500). At least, half of those calories came from Mega Mass.
There are gainers that are more protein-dense (i.e. N-Large2, Mass Fuel, Mass XXX, etc). I would recommend those over Mega Mass, unless you're an ectomorph that can burn calories rather well.