I started going down this route when I first started looking at arduino and RPi stuff. But, it is still hard to beat an Etrex 10. Of course, all I really want my gps to do is tell me my grid coordinate. Then I can check it against my map. Storing location data is pretty cool, and I will probably try that. Last Christmas, a family member gave me their old etrex 10 that they had never used. So now, it is hard for me to justify making one. Plus, it is in a weatherproof case.

However, I think you should make one. Why not? Give it a shot and tell us. I can almost guarantee that someone out there will benefit from your prototyping and code writing. I don't write code and I benefit from a lot of those types of people that write an arduino code for something, and I am able to take that and modify it slightly for my own specific project.