I have tried putting it onn my dash as I go through the toll lane and it hit about 50% of the time. They have your license plate info and will credit your acount if you have a toll tag, but after a while, I would think they might give you a notice.
One thing I have considered is putting the thing behind the license plate sticker on the window. It works in that area just fine. I try to conserve as much window space on the miata as possible.