Some of the Ikea stuff is very flimsy, but some of it is extremely tough. One of my bookcases is strong enough that I could hop up and down on top of it without any problems at all, and it would hold up.
Another recommendation - sometimes the plasticthingies they use to hold the shelves up are to blame - you can get strong brass hradware for cheap at home depot and that will help immensely. Also, when you're attaching that cheap plywood backing to your cheap $100 bookshelves, go ahead and take the extra 15 minutes to nail in all of the 50 or so mini-nails they give you. It will look better and last better.