I want to get one but would only use it when I ride for LONG distances. Aside from that I don't have a problem gassing up when it needs gassing up. Ya gotta when ya gotta.
An extra tank would be quite nice for peace of mind when riding cross-country for those long stretches without a gas station. But for daily riding I consider them an extra fire hazard, extra weight to carry around, and am curious as to how much of a shift in the center of gravity it would cause.
I suppose the deciding factor would be how much distance you ride on a regular basis. It may be more practical for you, in your particular use of the bike, to utilize one. If so then press on and let us all know how you like it!
Regardless, they're a nice option to have.