I have to admit that I use GPS as my main source of nav info. I use it in conjunction with Jeppesen flightmap. However I constantly monitor it against ground features (if VFR) as well as the normal radio nav aids. I feel the use of it in this way is justified because:
(i) It reduces my workload, particularly in IMC,
(ii) By virtue of the fact I can be more confident of my "exact" position difficult flight scenarios (flying in the Luton / Stansted area springs to mind) are less stressful,
(iii) I can manage without GPS if necessary. I don't always take it with me,
(iv) My setup logs a track file which is very useful when it comes to appraising how well you flew an approach / hold
I tend to be of the view that the more help I can get when I'm flying then the better. My decision to buy the GPS and associated equipment for the map display was made a few years ago when I was doing flights from Rochester to Blackpool early in the morning. The stretch from EGTO LAM BNN wasn't too bad because the VOR's are in the "right places" but I didn't feel confident in the area around Oxford if I couldn't get a service from Brize (parachute zones, HIRTA etc). I do stress though that I always have the VOR etc set up appropriately and prepare a proper flightplan etc. As I said at the beginning I use the GPS as my first choice but am quite happy to do without it if I have to.