At first, I wondered why we didn't see more maple syrup-based alcoholic beverages. Seems like a pretty great source for fermentables and flavor. Then I priced out a 5 gallon batch of maple wine and realized that maple syrup (the good stuff, why bother with the commercial bland crap you get a Trader Joe's) is *spendy*. Getting it for free or cheap would be pretty awesome.
Maple syrup is also pretty fantastic just by itself- mix enough into water (depends on the syrup, so you have to measure) to get about 1.075 OG (or higher), and ferment it either with a clean ale yeast or a champagne yeast. Gets all dry and woody, without needing oaking. Something that's on my to-do list is make a maple fortified wine- you can theoretically distill some of your maple wine and add it to another batch that's mid-ferment, catching it at 1.020 or 1.025 gravity. The increase in alcohol will stun/kill the yeast, leaving a strong but still syrupy and sweet drink. That's more or less how they make port. Of course, I can't distill at home, so this is a "maybe-someday" kind of project.
Generally, it's a lot of simple sugars and some flavors in there. Like honey, if you add it to the primary, it will ferment away almost completely, leaving behind the pleasant astringent woody flavors in the syrup but not much of the sweetness.
Using it as a priming sugar will leave a bit more residual sweetness in it, but limits how much you can put in. Similar to honey as well; you will want to pasteurize it a bit (unless you're getting it straight out of the boiler from the syrup farm you lucky somnabeech). I'd consider tossing it in at about 10 minutes from knockout. Don't want too many of those great aromatics to go away.