I have to agree with Rippers on this one (up to a point).
Whilst my phone integrates with my laptop for planning and storing routes, it isn't as 'user friendly' when on the move - mainly due to the fact that it isn't easily visible whilst driving (not mounted in car).
There is the obvious concern that if you are driving whilst using your mobile as a 'visible' satnav, then it could fall under the use of a mobile whilst driving and leave you open to questions from the Police - if you just leave it on the seat and listen to the directions, no such problems - but there is the temptation to 'just have a quick look' which could be illegal and, more importantly, dangerous....
...if it is in a proper mount, then no such issues.
On the positive side, my Garmin updates quickly and easily, and is very user friendly.
I like to be able to see what time I will get to my destination (maybe I'm sad, but I like it!)
I generally just keep it on the 'open map' setting without entering a route - so arguably it is unnecessary.
Either way, it's more useful than the ashtray
It was nice to have a SatNav when I was planning the route for last years Saturday Drive at Donington - using roads I didn't know, it is an easy process to make a map on Google Maps, and export it to the SatNav... (which didn't work in the end 'cos I took the red car with a basic TomTom which didn't import maps in that format

)