Ensure Plus

I'm not sure how many calories it has, but if you like it and don't have any medical issues around sugar or sodium I don't see why there would be an issue with having it.
If you want to make your own high cal/meal replacement shake you can do that too.. protein powder, full fat dairy, nut butter, banana, oats, you can even add ice cream if you really want to increase the cals.

http://community.myfitnesspal.com/en/discussion/10326769/are-you-a-hard-gainer-please-read/p1

This thread can be helpful. And while ensure plus has some good calories, you really need to figure out how many you need to maintain and eat above that.

I couldn't stand how sweet ensure plus was. Since I am always low on time, I was looking specifically for a meal replacement, with complete nutrition with a healthy amount of calories, so I went with a product called Soylent. However, if you are looking to add extra calories to your diet, just for calories sake - Nuts, Dried Fruit, Oats, Red Meat. Otherwise you could always get some Muscle Milk or something similar.

The product is specifically for those at risk of malnourishment. Usually given to those that have difficulties eating. Have you been diagnosed with that or is it simply a case of wanting to gain some weight? If you have been diagnosed then go for it, but if you simpy want to gain and have no other reasons I'd suggest working on eating more as a better approach.

My grandfather used it for a while in the last stages of his life. He could not handle solid food as well anymore, so needed the calories to not become malnourished. He used it in his coffee as a creamer. He too was complaining about the sweetness, but then again he was complaining about the taste of everything at that stage :-)