Reilly has probably played the best hockey of his career this season with Ottawa. 19 points in 40 games is nothing to sneeze at for a defenseman. He won't be getting the same ice time in Boston so he won't produce points to the same degree, but he is still another weapon for them.
He still has brain farts, but for the most part has been decent defensively.
I don't think its an overpayment at all. Can never have enough defensemen in the playoffs.