I've had a Snooper S4 Evo and the S100RLD for years. The S4 on it's own does GPS and Laser, the RLD fits under the bonnet and does radar and additional laser, though I've not fitted it to my new GTi.
It's always been pretty good, the radar attachment allows the unit to tell you if the Gatso is really working or if it's a fake/turned off etc. The only problem is it's quite old now and really slow to pick up satellites when you switch it on.
I'm tempted to change it for the Snooper 3Zero as it does GPS, laser and radar all in one unit.
I'd say I'm a safe driver and don't speed in particularly in built up areas, it's just nice to know there is some warning just in case you mess up and miss that camera. What's also handy is the GPS speed reading... it shows the true speed of your car which is slower than the reading on your speedo so on motorways I drive to the GPS speed not the speedo speed.