I brewed a Maple Brown Ale last fall (and plan on brewing it again). I added 2.25 gallons of Extra Dark syrup directly to the serving tank and filtered 7bbl of the beer in on top of it. I got a great maple aroma and a noticeable maple flavor & sweetness in the beer. I did some experiments to find the right concentration for the flavor and aroma I wanted and found that 2 gallons in 7 bbl of beer was a very faint aroma and slightly detectable flavor whereas 2.5 gallons was noticeably sweet (a little too much for me).
If you have a local maple sugar farm you'll be able to get the more flavorful syrup (Extra Dark). I don't think you can get it commercially....
hope that helps.
Brooklyn Brewery at the Culinary Institute of America
Hyde Park, NY