i've got a road angel compact and can't fault it tbh, had it about 2 years and not had a problem. Cos it uses gps it lets you know of accident blackspots and locations of fixxed camera. The idea being the police camera vans will only be operating where theres a accident blackspot.

I'd never trust it 100% though as you never know when theres a traffic car around the next corner so although it has it's uses it's not a replacement for your own common sense.