Basically what Lift2Live said, except that if you can get your cals through food it's better to do so.
Most weight gainers have about 10-20 grams more
whey protein than normal whey powder, so they have to get the cals elsewhere. Many of them just whack in a load of maltodextrin which can add fat quite easily if you're not careful with the rest of your diet.
I've taken them in the past though and have been pleased with the results. I don't have a lot of time to eat during the day, so I made the cals up in the evening post-workout. Prolab's Nlarge2 is quite good but can add the fat easily if you don't watch it. Also Reflex do a weight gainer which uses oats for carbs rather than malto. I've got some, it tastes foul, but it does the trick!

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Rich