Who's that?
As far as I'm concerned, fusion is the only conceivable power source that could enable our civilization to continue to progress for more than the next 500 years or so. As great as fission is, fissionable material is unfortunately a finite resource, too. On the other hand, if we can devise a fusion process that generates enough energy to 1) split water to "mine" hydrogen, 2) build and maintain the fusion plant, 3) power the reactor, and 4) have some useful energy left over to supply to "customers," then you have a virtually limitless power source.
Every pair of H atoms that convert to an He atom releases 4.8*10^-19 joules of energy. Annual energy usage in the world is approximately 5.2e20 J, so each year, we would need to turn 1.1e39 atoms of hydrogen into 5.5e38 atoms of helium to extract that much energy. Of course, we also have to extract enough energy to run the plant - let's say that actually consumes 99% of the power (only 1% efficient), so we'd have to tack on a couple zeros - we'd need to create 1.1e41 atoms of helium. Each atom of helium comes from splitting one molecule of water, so we'd "burn" 1.1e41 molecules of water each year, which is 3.2e15 kilograms of water each year. That sounds like a lot (and it is), but there are 1.4e21 kilograms of water on earth. Therefore, it would take us 430,000 years to use up all the water on earth - not to mention all the OTHER sources of hydrogen (hydrocarbons, other compounds in the earth's crust, etc). If we can get off of fossil and fissile fuels (several hundred years available at best), and on to hydrogen fusion, then we'll have given ourselves an extension of 429,500 years to come up with the next great thing - quite a difference!