My wife tried Sensa. She was suckered in by the "research studies" they did to validate the stuff. I read the research, which is some pretty bad science, and I cracked some jokes but she wanted to try it.
After about a month or two, she stopped using the flavor crystals and she admitted that they didn't work at all. They didn't affect her appetite or portion control, and they didn't lead to any significant weight loss. She's not terribly overweight, but (if I remember correctly) her stated goal is to lose about 20 to 30 pounds.
As an observer, I would conclude that a) the crystals are a bunch of snake oil nonsense and b) if you do see any success it's because you've successfully altered your own eating habits. In which case, why waste money on a product that doesn't do anything for you?