I dont know about those tanks specifically, but I will say its nice to have a little longer range, for sure. But introducing another system means that there is one (or two) more things to go wrong. I think they are kinda neat in a "huh..look at that" kind of way, but I vote for a larger tank.
Or...a couple of MSR bottles in the panniers. That's what I did in Mexico and the TAT this summer, and I only tapped into them once, and not for my bike.
Stopping at gas stations on a cross country motorcycle trip is one of the best ways to meet locals and get a flavor for the place. And as we all know, an sweet airhead (bike, that is) starts plenty of conversations around the gas pumps