3 UK have this functionality with some of their Motorola phones and I have to say it's a little rubbish. Time to Lock seems slow compared to a standalone GPS reciever despite the assisted part and when it does lock the possitioning is very vague and puts you in a general area rather than your exact location to within 10 meters.
Updating was also very slow and it would nowhere near be usable for car satellite navigation. (You would probably get away with walking, if it wasn't for the dodgy possitioning).
However 3 uk use it for finding local services, so in terms of a where's my local post office, cinema etc it works ok and will provide a map of the local area, so you could work out how to walk there.
I only tried this on a moto a835 and a925 so these may have been rather early models and GPS reciever quality may improve, but I wasn't impressed. I get much better functionality using a standalone bluetooth GPS reciever and 3rd party sat nav apps.