I worked for one of the biggest electrical stores in England for a few years recently before i decided i better get on with training and can say there is only one GPS Unit to consider, and thats TOM TOM!
We got one back in 12 months and that was a simple press the reset switch case. The worse GPS by far, Acer N35, We had them coming back daily. The Garmain units was OK and NAVMAN units have a poor database. Sony too have poor database but good features.
Tom Tom 300/400 and 500 are the best, unless you have one built into the car, and even then some inbuilt GPS systems are poor.