Unless you're blowing through a school zone with toddlers wandering all over the road I doubt a cop is gonna give you a speeding ticket, even then he'd probaly just pull you over just tell you not to ride like an idiot.

I wouldnt mind having one of those things on my road bike just out of curiousity of how fast I could go, but the novelty would wear off after a few days. The only other reason it could be useful is to say "I rode 106.7km today" instead of "I rode about 100km today", heh.