Those are good points. The biggest beef i have with OEM GPS is the cost. ~$2k for a GPS unit? I think for equal functions on a portable unit, you are looking at ~$300 (or less). The Garmin nuvi350 I have was $370 shipped (almost a yr ago) and has more features than the OEM unit. I totally love the SD card slot that I can load mp3 music files on and can use it as a walkman on a plane then when I get to the rental car, I have MY GPS unit and can leave the clunky maps at the rental counter.
With the extra money you saved with an aftermarket unit, you can get a second unit for hiking
and still have $1k extra.