Be honest; unless you or one of your buddies owns a sidecar motorcycle, how often do you see them out on the road? As for myself, I'd say not very often. So that means when I do, it's usually worth exclaiming about. "Hey, look at that!" or maybe "ooh, that's a great color," followed by a big smile and a wave if they're facing in my direction.
That situation may, of course, change dramatically based on where you're located in the world. I regularly saw quite a few more sidecars (usually of the utility variety) when I was in both Thailand and Vietnam, often loaded up with everything from propane cylinders to construction materials to sacks or crates of vegetables.
