As a mom of twins-- yes I have used them, and they're a great thing when needed! I'd like to see any of you who say how awful, cruel, sad, etc... they are try to keep two energetic little toddlers by your side at all times when you're by yourself. You're going to need one hand for something and that forces you to let go of one of the kids' hands. We took ours to WDW when they had just turned two and it worked great for standing in line. They could be with us, we didn't have to hold them all the time and they could enjoy some freedom. My boys are 5 now and still play w/ their monkey and puppy backpacks. (We haven't used them in years, but the boys think they're fun toys!)
So if you have a bolter, or an independant little one and you think he'd enjoy the freedom from a stroller, don't think twice and don't worry about what others think. You're keeping your baby safe.