The biggest benefit for me, comparing the built in sat nav to the Tom Tom I had running on my PDA, was the (lack of) start up time.
It may be that my PDA wasn't up to spec, or the version of TomTom has been improved with later releases, or it may have been an issue with my GPS mouse...
With my Tom Tom, I was often left having to sit in the car park / side of the road / etc for about 10 mins whilst it got a GPS signal and worked out where I was.
It then took another couple of mins to calc the route home.
Generally it dealt with tunnels ok - but got confused if there was slow moving traffic in the tunnel and I didn't get through it as quickly as it thought I should.
With the built in Sat Nav, it comes awake (and locked on to GPS) within 30seconds.
Route calcs take maybe 10seconds.
Tunnels etc aren't a problem, because it's linked to the wheel rotations )oe whatever other clever thing it is)
And - I don't have any mounting brackets / wires etc on display shouting 'break in - you might be lucky and find the Sat Nav system in the glovebox!'
HFT - mine works well.
Yes, there are some niggles, and it would be handy to
a) be able to use the voice tags on the phone to call
b) have incoming calls to display the number (or name if in the phonebook) that is calling
Every time I use it for the first call of the journey, it interprets my 'redial' as 'star 1 8' - no idea why
Hit end, say 'redial' and it's fine
so, couple of minor gripes, but still (imo) very worth while getting the combination