Although I consider GPS to be more useful than the steam stuff, I still rely more on the actual gauges than the GPS (if that makes sense). I've had two gps "failures" if you will. The first was caused by the GPS having a factory installed default of switching automatically off after 30 secs of no external power. During those 30 secs I was being a good pilot and keeping my head looking for traffic but when I checked the gps, it had turned itself off. That caused a major distraction on my part to get it back up and running and onto the right screen again (that setup has since been changed).
The second issue that I've had has been with poor gps signals. The only times that these have been an issue were while I was over the top with no ground visible below. The gps lost its signal for about 30 seconds.. then came back up for about a minute and then lost the signal again for 30 secs.
Both of these occurances have just reinforced my belief that the gps is a useful tool but not something you should rely solely on for navigation.
-Finn