I have two of the Billy bookcases with doors on the bottom half of both in the room we call an "office." I love these bookcases! They are MDF but much sterner stuff than the MDF from which other such bookcases are made. I have some heavy notebooks (I "do" genealogy) on the shelves of one and they have not sagged, as will and have other MDF bookcases I've bought.
My kids (DS and DIL) have a house full of IKEA stuff. They love it and it's very durable.