My mushrooms became soft, and slightly slimy to the touch, are they still safe to eat?

They are generally, and I stress GENERALLY, safe to eat. Store-bought mushrooms have a low risk of making you sick, especially if they are cooked beforehand.

Will they taste good is an entirely different matter though!

I toss them if they get like that, especially if the cap has opened and you can see the spines. Not worth risking.