This one has been discussed here a lot. My view is that while one can get some great solutions running on a PDA, a PDA isn't rugged enough for use as one's primary GPS. I've played with quite a few.
They have touch screens, and what happens if you touch the screen accidentally? I've seen products where the thing goes off into some unwanted mode and you then have to get the stylus out and fiddle with it.
Another factor is screenbacklight. Most PDAs don't have auto dimming, which (or at least very easily accessible manual dimming) ir essential for day/night flight.