My son has always had a Telematics 'Black box' in his car since passing his test, he's now 23 and we can't get rid of it quick enough!
I have been driving his car since i sold mine and have been waiting for the Model Y to be delivered. I find it stressful to be honest, you constantly have to watch the speedo, you simply can't speed. If you speed 3 times in a month your insurance can be cancelled. Time of day is a factor, you are marked down if you drive at rush hour or late at night. Smooth drive is a factor, it is impossible to reach a decent score - you find yourself hoping that you don't have to brake suddenly, pull away from a roundabout quickly or go over any speed bumps.
The worst part is impatient drivers constantly tailgating you because you are forced to drive at the speed limit. Every journey is the same, at some point someone will drive very close behind you trying to push you into going faster.
Overall I find it a massive distraction when you are driving, however without doubt it slows you down, as I said before you simply can't go over the speed limit at all any of the time, you are being watched and monitored whenever you drive.