Mostly depends on the game system. In D&D it comes up very rarely -- it's quite hard to run out of food if the group even half-heartedly tries to maintain supply. If it does happen, consumption depends on the critter. We'll generally avoid eating undead, oozes, fungi, and poisonous critters. Most groups will avoid eating critters with humanoid parts.
Depends on the creature type. Eating a grid bug gives you electricity resistance but lichen and kobolds tend to be poisoned. I ate a lich once and gain polymorph powers. Weird.