I can't remember who I heard telling about it- but the report was that the roads were narrow with no shoulder, traffic was heavy, and consequently, an island that was the perfect size and climate to ride around on was actually fairly sucky to ride on.

Edit: Here's where I read it: http://www.heptune.com/guam.html#Get...nd%20on%20Guam

Scroll down to the "Getting Around on Guam" part.