We've had a few people mention them in the past, but I don't think there's been a lot of interest. A few of the newer diet plans, such as the Sonoma Diet, suggest mentally dividing your plate, as well as using smaller size plates to help control portions.
I've been thinking about getting one because they're pretty, and because my poor old dh has no idea about carb, protein and veggie balance and it might help him be able to prepare food for me. He's an all or nothing guy. You get either a plate of meat, or a plate of rice, or a plate of veggies.........sigh