I recently bought a Nuvi 250W, yesterday came back from my third trip with it. The first two trips were just fine, but yesterday the Nuvi sent us out into the middle of a field. Came close to getting stuck in the sand. Maybe it's the map's fault (Europe), not the Nuvi's.
Earlier I borrowed my brother-in-law's PocketPC with AutoMapa (v3.8) GPS software. That was fun!! Instead of staying on the highway, it got me onto some super-rural road where I had to watch out for dogs, chickens and drunk locals in the middle of the road. All because that route was 100 meters
shorter than the highway. Since then, I use the 'fastest' option instead of 'shortest'.
The ecoRoute seems like a good idea. There is an EcoModder
blog and
thread about something similar.
One step further would be the GPS unit telling you when to accelerate, coast, etc. This was discussed somewhere here, but I can't find it now.