I had a guest medic riding my car recently. He was so proud of his GPS then we started getting calls. I was taking off for the calls because I knew where I was going and he would be punching info into the GPS. He kept wanting me to slow down so the GPS could give directions. The only problem was I had already made the turns before the GPS told us. His wanting to hold back for GPS instructions threw off the whole flow of the response, and frankly was confusing. I can see it as a useful tool for LDT's (long distance transfers) that our service does a lot of. For E calls though it is just one more thing to go wrong.
Our service just started putting GPS in out rigs. While it can come in handy, I still rely in a map and my knowledge of my local to get me there, and I don't leave the station untill I know where I'm going and how I'm getting there. I find it does come in handy showing you upcoming cross streets, specially when its hard to read the street sign or when the signs are turned or missing.