I think Garmin mobile Xt is great. The only problem is the GPS reception. I found that the Assisted GPS was not activated properly on the handset. Althought the server address was there, there was no access point specified. Since I added the access point, the GPS has been located much faster.
Try Tagggit. It's got a WLAN positioning system. It's free. It can get a fix in a few seconds and works indoors. I've been using it for the past week and it's as accurate as GPS. They have added a browser and they show your position using Google maps.
I think it's the first WLAN positioning system.
You dont have to use assisted gps. It just eats up your data limits if you use it a lot. Garmin gives very reliable turn by turn navigation (on its own) and ive found it very accurate. If you do want A-GPS for your phone you can turn on the one in Nokia Maps (but its not a very good system IMO). It does connect to the network (wireless, EDGE, etc) and the downloads may be small, but it does this every few meters to maintain accuracy.