We used o have them and there a pain i the arse, the gaffas seem to spend more time trying to catch you out than actualy doing any work. A mate of mine got pulled in the office for a bollocking for being late by half hour 1 morning, the tracker said he had left home at 8.33 and arrived on site at 8.37, The journey was 20 miles and the gaffa believed the tracker over the fact that you would need more than a caddy van to do 20 miles i 4 mins lol