The increase in HFCS has correlated with an increase in obesity in the US, but correlation is not causation. We've also had an increase in the number of people leading sedentary lifestyles, the number of people who own cars and drive everywhere, and the number of McDonald's chain restaurants. Correlation is not causation, and though the jury is still out on high amounts of HFCS, it doesn't look like it's much worse than anything else we're doing.
The whole thing is well worth the read, but the basic gist is that HFCS, by itself, isn't very bad. Its fructose/glucose ratio is only slightly higher than regular sugar and isn't something to worry about, and it isn't some kind of unnatural concoction like sucralose or splenda or asparartame.
Instead, the problem is basically a second order consumption problem: the stuff is fantastically cheap to make and use, which in turn makes sweetened goods fantastically cheaper to produce and sell, which in turns leads to fantastically increased consumption of sweets. Which in turn leads to fantastically increased waistlines.
Moral of the story: everything is good in moderation