I was told that the signal strength is comparable to trying to see a 20 watt lightbulb from 10,000 miles away!
This is probably true but one could come up with an amazing statistic like this for most forms of radio communication!
GPS can indeed be jammed. There is a very interesting report from the US Volpe Institute (probably there's a link on the www somewhere; I know it is definitely online in their archives) which tells you all about it, how to jam it, and how to make a system less vulnerable to jamming. All this stuff has been in the public domain for many years.
But we come back to the not-subtle difference between using a GPS for sole navigation (which is daft) and using it for primary navigation (which is perfectly acceptable because you have an alternative).
The 99.9% or whatever of the time that GPS works perfectly, it is absolutely worth having. If it goes.... well your cockpit workload goes up considerably but that's about it. The biggest problem with GPS is that we have an army of traditionalists who fear that the moment a PPL gets a GPS he forgets all his map reading skills. True for some no doubt, but true for all??