Your experience seems to be pretty consistent with most that I have seen. It's too bad, I had considered getting one also, I think I'll steer clear. I will continue to flash the brake light with the front brake lever, it is almost a habit for me now.
Thanks for letting us all know, and sorry it didn't work out. They do sell bulbs that flash when the brakes are first applied then go solid. But you were looking for something to activate on deceleration , weren't you?