Yeah, I have never seen a guy accelerate in full control of the puck like McDavid does, esp. in that he can do it while skating in circles or dipsy-doodling.
Besides him, the fastest skaters I've seen might be:
-- Paul Coffey: Had a similarly bizarre acceleration ability that seemed to defy the laws of physics, but didn't have nearly as many moves in the offensive zone as McDavid, and wasn't as good at stopping and starting.
-- Russ Courtnall: Could fly in one direction like a rocket... but couldn't control the puck very well while doing so. Average hockey IQ; hence, not even an All Star.
-- Pavel Bure: Similar explosive speed in one direction as McDavid, but he couldn't stay in control of the puck while zipping around in various directions like McDavid does.
Some other guys might enter the conversation, like young Mogilny, etc., but those three stand out to me.
Scotty Bowman said that Bobby Orr is the only player he has seen (and he's seen them all in person from the late '40s to today) who could beat other players with his skating like McDavid does. The thing is, though, even if he was just an average skater, he'd still have the elite offensive talent to score a bunch of points.
All that said, he's not as other-worldly as Wayne Gretzky was in his prime... but he's getting closer!