I have problems with sat-fixes on 640 on a regular basis.
Using the "Hardware Tests" app. I can see that the phone uses wlan networks or gsm transmitters as position source as long as there is no gps-fix. In a large city with many wlan networks and gsm transmitters the accouracy is fine then, so most people will not recognise they dont have gps.
This is what I have found so far:
- satellites are found very fast without data connection if the period of time since the last use of gps is short AND the location where you try to use gps is not far away from where you switched it of
- if you switch on data connection you get a quick satfix most of the time
- sometimes the gps-signal gets lost while using navigation (HERE, navicomputer). Signal is back after a long time, or when switching everything of and starting again (with data connection). Very annoying when you are driving alone.
The first two points are "typical" for a-gps i think.