Sometimes, something like that may sound good, but after the yeasts are done consuming the fermentable sugars in it, what you're left with isn't at all the flavor that you were expecting and can possibly turn out unpleasantly bitter.
That being said, there may be some folks out there who have successfully used it in their brew. I've just never talked to any.
I made a Maple Porter, and it turned out fantastic. I didn't put it in until right before fermentation was done. I put in a quart of Grade B. Once that fermentation was done, it sat in secondary for 4 months, and I bottled right before Thanksgiving. It has turned out fantastic, though being about 10% ABV, it has a nice maple finish on it, without being too bitter.
Add the maple at high krausen or 2/3 sugar break, and yes use GRADE B Dark maple syrup and youll get the most flavor and slightly woody tones out of it.