I just posted this about weight gainers in the supplement forum, but I think its applicable here so I'll just paste it.
For me, adding a calorie rich shake inbetween meals does the trick nicely. I think that the problem most people have when using a weight gainer is that they use it as a meal replacement, if you do this than it is no good as you are not increasing your overall calorie intake. If you are eating 4 evenly spaced meals a day then there is 4 to 5 hours inbetween that you can easily add a weight gainer shake without ruining your appetite for your next meal. I don't like prepared weight gainers as there is alot of empty carb filler and I really dont like the way they taste, but it is easy to make your own, my current shakes is
16 ounces of skim milk (You dont want the saturated fat in whole milk, we'll add good fat later)
2 scoops of protein powder (ON 100% casein, chocolate)
1 cup of maltodextrin (NOW carbogain, its only 14.99 for 8 lbs)
2 jumbo raw eggs
2 TBSP Olive oil
1 bannana
In total you get about 100g of protein and 1300 calories. It mixes to about 28 ounces and it is not as thick as you might think, its actually very drinkable, normally i just chug right out of the blender.
Good luck with your goals, remember you put on size in the kitchen, not the gym.