Oh god I am losing the will to live. Another GPS debate!!!
Have to agree with IO540 here, several hundred hours of GPS usage and never seen an error. I always fly radio aid, VOR/DME/NDB in parralell to the GNS430 which is connected to my altitude encoder (amongst many things) and they have always given the same story, unless of course the steam gauges are wrong as well........
Of course I always draw a line on my charts and have a PLOG (I need somewhere to put the R/T frequencies and QNH etc.
And of course when flying IFR I just draw a track on a blank piece of paper......