Google maps together with small scale (say 1:800 000 to 1:1 000 000) paper maps are good for the planning stage.

Gmap is good for making distance calculation simpler, paper maps are much better at showing size (and hence traffic intensity) of roads. I have found several locations in Europe where Gmap cannot accurately calculate the correct way (like this). It's a bit frustrating since I want to use Gmap-created routes on my homepage, together with elevation profiles..

So far, I've never used GPS while on tour. Real paper maps (not printed online maps) are preferred.

Added: I also add ~10% to all distances while planning.