I started looking into this because a health minded friend of mine mentioned it as a good alternative to honey, claiming that natural Agave nectar was just a beneficial and just as sweet, What I found was not so good...
First off don’t be dazzled by the word "natural"; U.S. food regulators do not legally define the term for anything other than meat, so it's left up to produce manufacturers as to its use.
http://www.foodnavigator-usa.com/Suppliers2/HFCS-is-natural-says-FDA-in-a-letter
Agave nectar is a "natural" sweetener that ranks relatively low on both the glycemic index and glycemic load scales. Sounds good - but the reason is because it has a high content of fructose. Fructose does not quickly raise blood sugar (glucose) levels because the body doesn't metabolize it well. New research suggests that excessive fructose consumption can impact liver function and promote obesity.
http://ajcn.nutrition.org/content/86/4/895.short
It turns out, agave has a higher fructose content than any other common sweetener, more even than high fructose corn syrup (HFCS).
http://www.webmd.com/diet/features/the-truth-about-agave
Bottom line - no matter what the form of sugar - even honey - too much is just too much!