You're right about him being athletic, but size is an issue. He's just under 300 pounds and with a 6'7 frame he's very lanky at that weight. He does have great technique, footwork and leverage, but he doesn't quite have the upper body strength or mean streak in the run game that makes a really complete tackle.
Unfortunately the left tackles don't get much better outside of Costanzo. There's more legitimate first round wide receiver talent, like A.J. Green, Julio Jones and Jonathan Baldwin, so I'd probably hold off on an OT until the second round.