Pain in the rear to count out the m&ms total) but everyone loved it so I would suggest it at any shower. When they are passing the bottle around to guess is a great time for people to introduce themselves.
Baby shower idea: Guess the baby's birthday! Great way to replace a guest book. You could use stickers of a pacifier and/or a rattle. Each person placed the sticker on the date they predicted and signed their name. Perfect to put in a scrapbook, too!