I see a lot of pallets here also. Mostly, I see oak, pine and poplar. I did get a couple of slabs of book matched ash a few years ago. If you keep your eyes open, there is a good chance you will find some good looking wood from time to time.
But, Bill is correct and most of the stuff is junk wood. In my line of work, many are stained with machine oil and other solvents, so they are worthless.