There's a lot of quality variability within IKEAs range as well. They have a bookcase line cheaper than Billy and you can tell it's there just so they can advertise something at a particular price point - I wouldn't want to own one. They have have a more expensive line made out of solid wood that is really good quality for not too much more.
I find that you can get a good feel for the quality of the stuff at the showrooms, since they get so manhandled by customers
As a designer friend puts it, IKEA is all in the edit. You can get some good stuff there, but you have to be careful about the design, the surfaces, and the finish of the materials. And you can't have too high a concentration of IKEA stuff in a room, or it looks too bland.
My library lives in 10 IKEA plywood bookcase units, about 20 linear meters of shelves, that is a really excellent piece. Simple design, all in birch plywood, and just the right strength (enhanced by using wood glue during assembly). It's been in use for 15 years now and still looks great.
I find that you can get a good feel for the quality of the stuff at the showrooms, since they get so manhandled by customers