Experts' Experts: Over the hill mushrooms — trash or treasure?

Long answer short: It depends.

From Cook's Illustrated:

Q. I have read that white button mushrooms taste better if they are just past their prime. Is this true?

A. Freshly harvested white button mushrooms have firm caps, stems and gills that are free of dark spots. That said, some chefs advocate the use of slightly older, blemished mushrooms, claiming that they are more flavorful than pristine, ultra-fresh specimens. To test this claim for ourselves, we sautéed two batches of mushrooms, one fresh from the supermarket and one showing signs of age after a week in the refrigerator. In a side-by-side comparison, the results surprised us. Tasters found that the older mushrooms actually had a deeper,, earthier flavor and were substanstially more "mushroomy" than the unblemished samples. This is likely because some moisture had evaporated and flavors were more concentrated.

The takeaway: There's no need to discard old mushrooms. In fact, their imperfections may actually improve the flavor of your dish. Do not, however, use mushrooms that smell fermented or look slimy.