We have the same situation. My wife and her girl-friend (or so I'm told
) have traveled together for 20+ years, all over Europe and other places, with my wife acting as navigator. As a result she thinks she's good at it.
She's not bad at it and is very conscientious, but she's not as good as she thinks, or as she once was (an analysis of my declining driving skills will not be a part of this post) so we find ourselves doing more head-scratching and re-tracing. I have also found that as years pass I am less likely to see all the signs that I'd like to think I used to see. Whether that is a sign of aging, tunnel vision or simply paying more attention to the driving is something I can't evaluate. A gradual but pronounced decrease in multi-tasking functionality, the modern behavioralists might say.
In addition, the signage isn't all that good in many places, especially with the proliferation of orange cones that seems to precede us on every trip. The GPS has solved a lot of those problems for us. Being able to see our position on the moving map is a huge advantage, and there are no edges to the map so we don't get lost while shuffling pages. Bottom line for us is that we don't leave home without it.
My profound grasp of the obvious also causes me to notice that the most popular option on the new luxury vehicles is the factory installed GPS navigator. Maybe there's a reason.