GPS's and Rechargable batteries?

I am planning to purchase a handheld GPS unit and was wondering about the units that have built in rechargeable batteries. 

The integrated rechargeable battery looks very convenient to use, just plug the whole unit into the charger either at home or in the car and the unit is good to go for ~15 hours when you are ready to be cordless.  The downside looks like unless you have a spare rechargeable batter, your GPS is useless when the charge runs out.

I notice most of the units available use regular disposable batteries and this must be for good reason.

Brian 3Dog

My gps's are old and use AA batteries. I like to use the rechargeable type and keep an extra set with me.
I can also buy AA's if needed.