These devices don't track and log speed. They DO track braking forces however (how hard people brake is a better indicator of poor driving). They also track time of day use and miles travelled.
What's always bothered me is that if some douche cuts me off, I slam on the brakes to avoid a collision that is 0% my fault, then MY insurance goes up (relatively speaking). That's just one beef.
...bigger beef is that I would feel like I'm limited on how/where/when I drive my car. Sh!t, I'm out of eggs and dinner's in 30 mins. Guess I should go out and get some eggs. Oh wait, it's rush hour...those $4 eggs are going to cost me another $15 in insurance this month. Fak. Obviously not an exact science, but I don't like it. I don't like feeling like I should run yellow lights, etc.
I think the potential for savings is outweighed by the potential for abuse. I'm concerned that in a few years the "discount" will fall off and my rates will just go up.
The one nice thing would be that it'd be really hard for the insurer to claim "material misrepresentation" if you say you drive 15km each way to work and then they use google maps to determine you actually drive 16km (yes, folks, we have a client being denied insurance because his work was actually 1km further from home than he told them it was).