I haven't used the built in nav software but I have got TomTom 6 installed and it works just the same as it does on a windows mobile phone like your XDA. I think it's pretty much accepted that Tomtom is better. I don't like the built in GPS, compared to my Holux BT receiver it takes an age to lock on and my position kept jumping all over the place.
If you want to see what I mean about the GPS lock time and accuracy, try installing Nokia Sports Tracker (free, google) and go for a drive. When I exported my route to Google earth it told me I had travelled through houses and fields. With the BT it was perfect.
I would recommend Tomtom and a BT GPS.