I guess I'm with Keith here, most of what I've read and heard from speaker designers suggests that MDF may be the best for the price, but otherwise has a number of disadvantages.
It does have a LOT of disadvantages. The advantages are it's cheap, easy to machine, and it very flat so it can be painted.
If you're ever in a higher end woodshop, throw a piece of MDF on the vacuum table CNC machine. Air passes through it like it's not even there! Just about every CNC operator uses a sheet of MDF as the wasteboard, a piece that goes under what you're cutting that can be thrown away once chewed up. Reason being, air passes right through it and the vacuum will still hold down the board on top of it.
Other bad about MDF - doesn't hold a screw and screws loosen over time, it's very compressible (very low Rockwell hardness) and mounting drivers to something soft/squishy hurts transient attack, low tensile strength needs to be compensated for by laminating multiple layers together and once you start mounting drivers in 2-3" thick baffles the tunnel they're housed in starts to resonate.
... since I'm a manufacturer who doesn't use MDF, feel free to take that all with a grain of salt.