As a first year medical student who never really learned how to cook during college, I found myself skipping meals to divert time from cooking toward studying. I'm currently using ON's Mass Gainer as a meal replacement rather than.. well, a mass gainer. One "serving" (two scoops) and you've met your daily nutritional needs for most vitamins and minerals; when mixed with two cups of whole milk, you've just added another 300 calories with some healthy fats and another 16 grams of protein. Its also got some ingredients of other fairly common dietary supplements included such as creatine, choline, and inositol to help with energy throughout the day. The only gripe I really have with this product in terms of nutrition is the excessive amount of maltodextrin. While it technically is a complex carbohydrate, it yields a glycemic index fairly close to that of pure glucose. I put "serving" in quotations because two whole scoops of this stuff, even when mixed with water, is just too much in one sitting for me. I typically have three scoops throughout the day as an incredibly convenient meal replacement saving crucial time to study and ensure that I'm never late for class. The chocolate taste was great to start off with, but after about 30 some pounds of this stuff, it does start to get a bit old. Adding a banana, peanut butter, coffee/espresso, even strawberry milk help give a little variety. I guess it does mix fairly well with minimal clumping, but powdered foods are never really clump free. I use a shaker bottle with a mesh insert that I've found to suit my needs, but others might prefer a blender, especially if adding other ingredients.

All in all, its cost effective, time-saving, nutritious, and tastes good enough for me.

I have Crohn's disease and gastroperesis, and therefore can't gain weight to save my life. One of my friends recommended this product, and I'm hooked.
I can only stomach one scoop, because they're massive. But mixed with a cup of coconut milk, a banana, and 2 scoops of peanut butter, gives you a 1,200 calorie shake. I drink one for breakfast every morning, and it's helped drastically. It also makes me more hungry during the day, helping to add even more calories.
I've gained 5 lbs in two weeks, which is substantial considering i haven't gained a single pound in 3 years.
The only side effect is, it has a stimulant effect on me. Now I drink a pot of coffee a day, but after this shake I feel completely wired. So I'd definitely recommend drinking before a workout, and not before class like I keep making the mistake of.

The serving size is huge and much of the powder clumped on the wall of the blender jar. I mixed one scoop with whole milk and still felt it in me the following day. I will finish the jar using smaller quantities as a daily "snack".
My go-to protein powder is Jarrow unflavored whey protein. An equivalent amount of the Jarrow Protein would deliver slightly more calories than this ON Serious Mass (but it would be more costly and it is not the same nutritional profile).
ON is a good brand, but I would guess that there are better choices for weight gain

I was 35 years old 6' tall at 111lbs, I was like that since i was a teen struggling to gain weight. I went to doctors and nutritionist to find a way to gain, but they all said it was genetics and I had a poor diet and a high body metabolism. Though I worked on my diet and brought it up to standard I still did not gain anything much (1lb to 2lbs on and off). I tried other products from Universal, HealthyNfit, etc... nothing worked. So I came across this product Serious Mass from Optimum Nutrition. So I tried it, at first I was using half serving per day with whole milk and wasn't gaining much at all or seeing any significant difference. So I opted to 1 full serving per day, after 19 12lb bags and a 3 days per week, 1hr per day workout (weight lifting, short reps high weights) I moved from 111lbs to 186lbs (some days I skipped my shake, due to my hectic work schedule). I stopped the product and gym for about 6 months due to a shoulder injury. I went down to 157lbs where I remain to date. Be aware it can mess with your mood for some people for about 2hrs after consuming. It is important to see a doctor before using this product. I must say it really worked. It boosted my self confidence A LOT and even made me look very young as well!! ;)

This was the very protein powder that I ever purchased. I make my shakes with almond or coconut milk. It tastes just like a banana milkshake to me. I have tried other fLavors with this brand, but the only one that I really like is this one. I also use Testorex "“ Boost Testosterone Levels in Days, While Decreasing Estrogen and Cortisol Levels for energy and mass. I haven't had any side effects from using it .