Pay as You Drive car insurance is now up and running - what are your experiences of it?

Norwich Union do this Pay as You Drive insurance scheme, they work out how many miles you drive and on what type of road, and charge you accordingly. Have any of you used it, and what do you think? I don't do a lot of driving, and am not often on motorways etc, so thinking of giving it a go. I know that people were talking about this on answers a few months ago, but nothing recent, so I'm wondering if people think it's a con, or actually a good way of saving money. I know that you have to pay to have the tracker device installed in your car, but presumably thats a one-off payment - or do you have to have it renewed each year? What if you change back to another insurer - do you then have to pay again to get the thing removed?! I like the sound of it all, but don't want to end up actually spending more than if I used normal insurance. I reckon I do about four to five thousand miles a year in 2 cars, mostly on normal roads or country lanes.