Hey Kevin:
The service is free - the signals are broadcast from satellites 24/7. Get one that is WAAS (Wide Area Augmentation System) enabled and it will put you right on a brushpile or even a big stump for that matter.
I use a Garmin Geko 301 for pinpointing my brushpiles. It doesn't have mapping on it - only the waypoints that you save. I also have a Garmin Etrex Legend that I use for navigation. It comes with a basic map that has a rough outline of the lakes I fish on but probably not small lakes. It has saved my butt in heavy fog a few times and it does a good job of pinpointing brushpiles too but I like the Geko better for that because it updates faster. The Etrex has major roads and highways on it so you can use it for travel as well.
I have some friends that have the Geko 201 and they are happy with those units as well.
A good way to learn about GPS and to compare units is to go the Garmin website - Garmin. They have a good description of how GPS works, a place where you can compare the features of their various units and even read the owner's manuals for each unit.
A GPS with WAAS will save you a lot of time and trouble getting back to your honey holes and they are well worth the expence. You can get the Geko 201 for around $120, the Etrex for around $160 and the Geko 301 for around $200.