I used to be a real doubter but changed my mind when following someone through the Dublin backstreets when we were late for the ferry- we would never have made it if we had to keep stopping to read maps.
I only use mine when I need to- generally in foreign cities where I just want to get through asap, or if I need to find a specific address.
I dont bother with earphones and can happily concentrate on my riding and the surroundings, because it flashes before I need to turn, and it becomes just another instrument that you subconsciously read without staring at it like your speedo or tacho.
There is a funny story though about a German guy who crashed his bike on the infamous Nurburgring. When questioned by the police in hospital, it turns out he actually intended to go to Nurnberg on the other side of Germany but programmed the wrong place in on his GPS!
Guy