I used drywall for the base of my current layout, and I love it, easy to work with, just need a utility knife to cut it and alot cheaper. My layout is two levels the upper is made out of 1x3's with 16" centers then drywall screwed down. I used Woodland Scenics foam roadbed, which is held down with carpenters wood glue, then the track was nailed down. The current layout has been up for 4-5 years now, no sags or any problems what so ever with it. If I ever build another layout, I WILL be using drywall.
Just remember one side of the sheet has a tapered edge, face it down, so the completely flat side of the drywall is facing up.