Having the ability to drive 1500 miles without having to refuel, is massively cool.
And as someone said before, it's all about having options.
For example, the argument that you'll be carrying "500/600/700/800/pick one lbs" extra weight is a bogus because there is nothing preventing you from NOT filling the aux fuel tank unless you want to.
When I'm around town, I put about 30 gallons per month and I don't keep much in my 90 gallon aux fuel tank, so the only dead weight is the tank itself, about a 100 lbs or so, whoopie.
Last year, while on our summer desert trip, we spent several days driving in CA; however, we didn't have to buy not one cent of their overpriced (+50 cents higher) fuel. Hahaha, they got screwed! Yet, when fuel is cheap, you can stock up.
One recommendation I would make is to add a fuel water separator filter to your configuration. Adding another fuel filter is another HUGE advantage to having an aux fuel tank.
Here's how I did mine:
http://www.rocketcityrockcrawlers.c...talling an Aux Fuel Tank paper 2007-06-18.pdf
And here's a pic of what my aux fuel filter saved me from in the last two months. Dirty fuel from the aux fuel filter drain (left) vs. clean fuel from the HFCM (right).
Hope this helps.
Al