Definately a must have in my opinion. It seems like the few times I used to forget them I'd end up with a piece of dust/dirt in my eye that would bug me for my whole trip. I never forget them anymore. The amount of times I've had flies and bees bounce off my sunglasses helps me to never forget them anymore.

For $50 at the LBS I got a pair with removable lenses that came with four different sets of lenses from low light lenses to really dark ones. It was totally worth the money.