My only experience with mushrooms is the analogy between them and the American population. "Mushrooms are fed a lot of crap and kept in the dark."
I read in an old Organic Gardening Book that mushrooms were heavily sprayed with chemicals and to avoid using the soil or growing medium due to its toxicity. This O.G.B. was written in 1978, and I don't know if mushroom production methods have changed since then, or if they still use chemicals on mushrooms. If your potential mushroom compost supplier is local like you say, ask him what its made from and whether or not its been sprayed.
Home made compost works great and you will have a better performing, nutrient rich garden because of it. My own recipe consists of 1/3 horse manure, 1/3 rabbit manure mixed with rice straw and 1/3rd raked up maple leaves which break down over the winter months into PH perfect topsoil and leaf mold. This is, labor wise, the fastest and most convenient nutrient rich growing medium I have come accross and all the ingredients are free. The only problem is alfalfa sprouts coming up from the horse manure and these must be hoed and kept down or yanked out by hand while they are still young and easy to pull.

In planning my garden beds for this year, I called the county extension office for advice on where to obtain organic resources. The guy was friendly, I'll admit that, but when he suggested I go to the municipal waste treatment plant for "all the free compost [sludge] you want, I politely said no thanks. Numerous studies have shown that this stuff is not safe for administering to food gardens - in fact, I wouldn't use it anywhere on the property. It is IMPOSSIBLE for the treatment plant to leach out all the heavy metals and prescription drugs and I will not put that crap into the soil. He did give me another resource, though, that I will probably take advantage of, a local mushroom house also has free compost to give away - and it is organic. Does anyone have any experience with mushroom compost?