(More often then not) to me says that he’s saying it doesn’t work more times then it does, that has to be over 50% of the time. I question this statement.   
Well, maybe, but I've sure seen a lot of GPS failures in mountainous terrain.  Down in the bottom of gullies or deep canyons and under heavy tree cover seem to be places where failure to get a fix occurs.  
Recently when I was on Marion Mtn (10,600+), the GPS pointed us to a false summit.  GPS's are frequently but not always good.  I can't say what percentage of the time under what conditions failures occur, but they're very common.  I get GPS failures multiple times 
every hike I go on.
Roarmeister, anything to add?