http://www.slashfood.com/2007/02/18/what-is-agave-syrup
If you check out the labels on natural food products, you'll see agave syrup listed as an ingredient more and more often. Agave syrup - sometimes called agave nectar - is a natural sweetener that is marketed as a healthy alternative to processed sugars, as well as an alternative to honey, corn syrup and similar liquid sweeteners. The syrup is made from blue agave, a type of succulent, cactus-like plant that is also used in the production of tequila. The thick liquid comes in light and dark forms, the latter being unfiltered, and tastes similar to honey.
So what makes this syrup "healthier" than other sweeteners? It has a very low Glycemic Index (GI) value, so although it is noticeably sweeter than sugar, it doesn't cause the same type of "sugar rush" that other sweets do. Table sugar has a GI value of 68, on a scale from 1 to 100, while honey comes in at 55. Agave syrup has a value of around 15.
Agave syrup can be used in baked goods, with a reduction in the amount of liquid in the recipe to account for its presence. Starting with a recipe that already calls for honey and substituting it in is a good way to start, but as a general rule, you'll will need to spend some time playing around with your favorite recipes to get them to work, especially if you want to substitute it into a recipe that calls for crystallized sugars. The syrup will dissolve easily in liquids and is a good choice for sweetening iced tea or lemonade.
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Agave nectar may substituted for part or all of the sugars or liquid sweeteners in many recipes. Drinks, salad dressings, sauces and many desserts are among the easiest substitutions. More experimentation may be necessary when substituting for sugars in recipes containing precise chemistry - for example, cooked candies and some baked goods.
Candy recipes like toffees and nut brittles rely on chemical reactions provided by refined sugars which may be disrupted by substitutions. It may be possible to substitute, but ratios could take some tinkering to produce optimum results.
Similarly, recipes for baked goods containing white sugar may be too sensitive to changes in the moisture level of ingredients. If replacing all the sugar in a recipe (while reducing liquids) does not produce good results, try replacing only half the sugar with agave nectar.
Honey: Replace each cup of honey with one cup of agave syrup.
Maple Syrup: Replace each cup of maple syrup with one cup of agave syrup.
Brown Rice Syrup: When replacing a cup of brown rice syrup, use 1/2 to 1/3 as much agave, and increase other liquids in the recipe by up to 1/2 a cup.
Corn Syrup: When replacing a cup of light corn syrup, use 1/2 as much agave, and increase other liquids in the recipe by up to 1/3 of a cup. Like corn syrup, agave nectar will not crystallize.
Substituting Agave Nectar for Granulated Sugar
White SugarFor each cup of white sugar replaced, use 2/3 of a cup of agave and reduce other liquids by 1/4 to 1/3 cup. This substitution will also work for Demerara Sugar, Turbinado Sugar, Evaporated Cane Juice, or Sucanat.
Brown SugarFor each cup of white sugar replaced, use 2/3 of a cup of agave and reduce other liquids by 1/4 cup. Because the moisture content of Brown Sugar is higher than that of white sugar, liquids may not have to be reduced as much when substituting agave nectar.
Other ConsiderationsAgave syrup may cause baked items to brown more quickly, so reduce oven temperatures by 25°F is and increase baking time slightly.