I'll be following this thread as I have the same question.
I had considered just using my iPhone and Google maps, but there are plenty of places where I won't have cell coverage and I would imagine I'd use a significant amount of data as the software continuously downloads additional map data.
I've looked at the motorcycle-specific GPSs and am disappointed at the price premium they command. I'm pretty much a fair weather rider so waterproof isn't really required. I'd plan to toss the GPS in the frunk if the bike was going to set outside unprotected for any length of time or in inclement weather.