I agree that NiMH batteries are the way to go. I use Rayovac 15 minute rechargables and have had no complaints about them. I usually plug the GPS in while in the Jeep because I like the backlight on while it is in the mount. If I plan to be out all day sometimes I will carry a spare set in my pocket, but so far I have never had to use them.
Rayovac is supposed to be coming out with a NiMH that will not discharge by itself soon. I'm sure other battery companies are working on that, too. Those should be cool.
On safety, do not carry spare batteries in your pocket with keys or change. That can give you a "hot pocket", which I discovered a couple of years ago!