A friend surprised me with this when I woke up today & saw the gift in my e-mail. I already had Jamestown, Bit Trip, and Cave Story+ but am really enjoying the others in this pack!
Shank is a beat-em-up with a surprisingly smooth & variable combat system. Animation along the lines of Samurai Jack, from Cartoon Network.. and probably my least favorite part of the game (though it does flow smoothly)
Havent tried Super Meat Boy yet but I've heard good things.
Gotta say the real stand-out for me in this pack is Jamestown though. Its an excellent shmup, and the only downside in the game is a lack of internet Co-op. Local Co-op on a big screen TV (& arcade sticks ^_^) is an absolute blast with 4 players though.
Havent tried Gratuitous Space Battles yet, but it seems that you are actually given two copies of the game if you activate it in steam. One key is given to you on the Humble Bundle page, the other is given to you in Steam itself when you launch the game, and both are different, allowing you to create 2 separate multiplayer accounts to use. (thats as far as I got in the game though)
Best humble bundle yet, no doubt. I hope they break $1,000,000 in sales by midnight tonight. I think I'll buy a couple more for friends to help that become a reality.