I agree with @Harvey, this season is all just about having him here.
I watched a fair bit of him for Sheff U via net highlights, and obv spoke to the Sheff U lot about him, and he's a huge upgrade on Brice. Different class, which is what we need for the Prem.
Dunno if it's been posted already, but this is a cool vid of some of his moments...
In terms of agility he undoubtedly is, but being a keeper is also about consistency (something Samba struggled with), relationship with the defence, gathering difficult balls to relieve pressure, personality, reading the game and launching attacks - areas Samba often excelled at.
I haven't seen enough of Henderson to know how he'd do in that regard.