From what I can tell, it seems that the only advantage is that it has a lower glycemic index.
Here's my logic...
On my organic blue agave sweetener bottle, it says that it's 25% sweeter than sugar, so you can use less than sugar (use 3 parts of agave sweetener for every 4 parts of sugar).
Cane sugar has 4g of sugar in 1 teaspoon (4g). (4 teaspoons (or 16g) of cane sugar = 16g of sugar).
Agave sweetener has 16g of sugar in 1 tablespoon (21g). (3 teaspoons (or 21g) of agave sweetener = 16g of sugar).
So from a nutritional point of view, if you substitute 4 teaspoons of sugar for 3 teaspoons of agave sweetener, you are getting the same grams of sugar (16 in this case), which has the same amount of calories (64). So there seems to be no benefit here.
Are they just trying to say that you're using less of the product so you can save money? If so, that is very misleading, since agave sweetener costs significantly more ($0.16/oz, $0.04/tsp) than sugar ($0.05/oz, $0.007/tsp) by 82.5%. That's nowhere near the 25% it can cost more than sugar and still be a cheaper alternative.
The only other benefit I see is that it says that its glycemic index (39) is lower than sugar's (68) [source].
Am I overlooking anything? Are there any other benefits?