It's not fair to generalize something to death.
MDF and particle board are not the same. Not even close. Particle board got it's bad name starting 50 years ago for good reasons. GRJ has mentioned a few, including sagging and water sensitivity.
MDF's close cousin, MDO, or Medium Density Overlay, which has an MDF core and a tough paper overlay, is used extensively by state highway departments for road signs. It paints nicely and holds up very well under adverse weather conditions, so water isn't a problem.
MDF can sag over long distances, but nowhere near as much as particle board. With structure underneath on 16" centers it will be fine.
Neither particle board nor MDF are currently made with formaldehyde. It was phased out at least 10 years ago after it was discovered to be a problem in sprayed-in insulation.
Like the baltic birch MDF machines very nicely, with sharp edges, perfectly flat surfaces, and with one advantage over birch-- the ability to accept very small details, via router, carving tool, or small saw.
John, the bookshelf in your photo needs to be made out of MDF, and supported underneath it's front edge by an MDF stringer (1x2 dimensional, or 3/4" x 1-1/2" actual) with it's wider dimension oriented up and down. Do that and it won't sag.
I've used MDF extensively without any issues. You can have zero problems with it too, as long as you use a little common sense. (Contrast this with particle board, which nothing can save).
Mike
