This isn't a great move, but it isn't a bad one. McDyess is solid. But I have to ask myself if this forces Duncan to the perimeter more, to let McDyess work in low. Wallace, in my opinion, was a better fit for San An, and when you get right down to it, so was Shawn Marion.
There's no question the Spurs will be better next year, with the addition of Jefferson, but I really think how far the Spurs go in the playoffs depends largely on whether or not Ginobilli is there in the final minutes of the game and what he's physically capable of doing. And, of course, without Duncan, the Spurs might has well just go home.