I've been reading around this subject as I'm planning to get a GPS of some description sometime soon. Issues with mobile phone based systems seem to include battery life, waterproofness, screen brightness/contrast not good enough for use "on the move"
At the moment I'm leaning towards etrex Legend HCx rather than one of the bike specific models.
Not wanting to plug a "competitor" forum, but there is a LOT about GPS over at yacf.co.uk, you may want to have a look?