The posts so far don't seem to address the basic reliability and accuracy concerns with GPS. Without preprogrammed RAIM monitored approaches (which exist in many places for non precision type letdowns) the use of the system entails an unpredictable risk. Likewise with holding if there is a concern with traffic separation or terrain clearance (as in protected areas).
GPS is great as a backup tool and for assisting with the initial above MSA tracking into the often poor range of an NDB .... but, without the added reliability of RAIM and DGPS for more precise approaches, it is a bit of a suckerbait system.
It horrifies me to see GA pilots here program their own versions of the published letdowns .. and then proceed to fly them ....
[This message has been edited by john_tullamarine (edited 04 July 2001).]