I posted months ago about Agave Nectar on a cooking forum I belong to, this was a reply from a nutritionist who posts there as well...
On the Glycemic Index (GI) everything is compared to the amount that glucose elevates the blood sugar level - this is set at 100. The starch in things like potatoes and rice (which is actually just long chains of glucose molecules strung together) hits the GI at 95. Just below that at 90 is Honey and sucrose (table sugar). Just for grins - a banana hits the GI scale at 60. Fructose, which is not directly absorbed but metabolized in the liver before it can be used, is only 20 on the GI! GI values stated here are from On Food and Cooking - The Science and Lore of the Kitchen (revised edition) page 659 - by Harold McGee
Here's how this works: Since fructose is twice as sweet tasting as glucose, you need less of it to achieve the same degree of sweetness. This is good because it means you are consuming both less sugar as well as less calories - and it has less immediate impact on the glucose level - and thus a longer lasting supply of energy without the initial "sugar rush", and it reduces the amount of sugar the insulin system must deal with at one time.
Now, here is the kicker - it's basically the same thing as High Fructose Corn Syrup (HFCS) that some people want to paint as being an evil thing, in spite of the facts that it is better and safer for diabetics, children, or when you need a long sustained source of energy. It's just from a different plant - the agave instead of corn ... or in other words, it's basically High Fructose Agave Syrup (HFAS).