I love the free 3D movie and the factory-themed area you walk through to get to it. The movie is pretty funny and enjoyable.
Yes, the wall of M&Ms are overpriced. I still love that you can get exclusive colors, mixes and prints here. Last time I was there I stocked up on "Viva Las Vegas" candies with Green (the girl M&M) on the back of them. I remember back in 2000 they had a special Millennium mix of black, white, silver and gray. It was awesome.
It's a definite must-see, especially with the family. You don't have to buy the full $13 pound. You can just get a tiny bit if you can resist the temptation!