IKEA is full of cheap fiberboard (cardboard) junk.
Their mattresses are awesome though, at least mine is. I checked and I have the Sultan Fossing King Mattress. It is comfortable as hell (although I didn't try out a temperpedic or the other really good expensive one)
If I was you I'd at least go try check the IKEA mattresses out. Another neat thing, is that the IKEA king size foam mattresses come in a rolled up compressed bag so they're very easy to transport.