I have used GMXT on my N95 for two years now and just recently moved to N97.
On my bike I use the best-of-the-best, a Garmin 278. My wife use a Garmin 760 in her car.
I use GMXT on Nokia N97 in my car and when walking. The N97 is, by far, the quickest to find satellite contact and has the best map resolution & map detail. The main advantage is that I have it with me. Always.
All three devices accepts upload of waypoints and routes. This is, for me, an absolute requirement for a GPS. I can't accept keying in addresses for routing. I always create a set of waypoints prior to a trip (local, to USA, to Rome, etc) and upload them to the GPS. Typically a few hotels, customers to visit, alp passes, etc.
For usage on my offroad bike there is no serious alternative to the Garmin 278 and route planning in Mapsource. Therefore I'm stuck with using Garmin also for other applications - thus no alternative for me to use GMXT in my mobile.
And why not? Since the maps are stored in the phone memory the network costs are neglectable. Roadmaps (City Navigator) are great and both topo maps and blue charts are available for most areas worldwide.
Only negative experience with GMXT is that neither the N95 nor the N97 are very reliable. Crasches and need restart too often.