I might be old school on this, but I firmly believe you need to know your response area well, and cannot rely on a GPS unit for every call. With that said, I take a portable GPS with my on my shifts, but usually only for long calls.
The first few weeks you work in a new area, you should have the map out all the time. If you are working non-emergency, find all the facilities and homes you go to ofen. In your posted time, find the closest hospital to each of them. Then find the easiest way between them. How do you get from each posting spot to these facilities?
Your truck should be equipped, at minnimum with a good street map of all the areas you cover, and a street guide. Learning to use a street guide takes a little while, but once you get it down, helps enormously with navigation.
You may consider buying yourself a good street map of your own, and circling common
destinations for easy reference.
You first few months at a company should be spent teching, learning the paperwork and protocols, etc, and with all your time in the passenger seat, learning the roads. You should always have a situation awareness of where the closest hospital it, and the easiest way to get there.
With all of that said, GPS units are great for long trips or extremely obscure streets (but the street guide is better).
Last christmas I bought myself a GPS, I think it was the bottom of the barrel, but it works fine. My new phone (Droid) has one also, which actually works much better, so I have been using that for long trips.
