Diabetic here. I would wear the contact lens in a heartbeat. The idea is that this device would replace the finger pricks, otherwise known as holes in skin. And when you repeatedly test on your fingers (6-10/day), that's a lot of holes and a lot of blood. There is risk for infections, scabs and blisters. And long time diabetics develop callouses on their fingers from testing which means that they need to poke deeper to get blood.
Plus the checking isn't really "constant." You have to periodically check during the day. That means that you can go high or go low between checks and perhaps not realize it until you have symptoms.
There are constant glucose monitors. Essentially a large pager you carry around that is connected to the body via an injection site. They're great. They measure changes in BS very well, but they're very limited in duration, insertions are painful, and the disposable bits are really expensive. You also need the finger sticks because (as far as I'm aware) few are accurate enough to give you the same level of accuracy.