For the record I'm a GPS fan. If I've got a panel mount it's my primary nav facility and it works more reliably and more accurately than anything else on the panel including the FM Immune VOR/ILS (which works no better than the none immune one but that's another story)
The only caveat on GPS is the issue of relying on the database to be both up to date and accurate when it comes to airspace (they usually are if regularly updated but not always) so there's a need to xref a current chart and NOTAMs but that's routine flight planning ain't it???
For hand helds extra caution is required because they will lose coverage from time to time depending on how uniform the view of the constellation across the sky happens to be. Early morning East bound near Luton seems to be a problem at present in this respect