My first year or so of decorating, those pudding cups were pretty much all I used to fill my cakes, since that's what my wilton instructor suggested. Now I have ventured off into making scratch fillings, but when I'm making a cake for a favor or in a hurry then I still use those pudding cups. They work great!
Remember though, although the pudding doesn't need to be refrigerated before it's opened, it's supposed to go in the fridge after being opened... I didn't know this for the longest time and left my cakes sitting out for days!!
BTW, I used the instant pudding as filling for the last cake I made, the jell-o brand in the box, and it was really good! 1 box makes a good amount too.