NinjaBee shipped A Kingdom for Keflings back in 2008. I tried the demo and was hooked, so bought the real game (it is an XBL Arcade game). Other things came up at the time, and I never finished it. Then I got back to the game in September or so and played it through.
Then I saw news that a new Keflings game was coming out. And even better: Major Nelson said that it was going to be part of a holiday series of sales promotions! Just 800 MSP (aka US$10).
I bought A World of Keflings the very day it came out. My wife and I played local coop through the entire game over the next couple days. It is simply one of the best games for this. Most local coop games have one person in charge of the play, and the second person participates in some limited fashion. Not in World of Keflings. Both players are in charge. It does an amazing split screen when the players are distant and then seamlessly merges the two panels into one, when the players are in the same world space. It is completely fluid and does not disrupt gameplay.
There is a fun story and a great sense of humor throughout the game.
This game is a must-buy, whether you are playing by yourself or with a partner.
(and if you're an XBL friend of mine, then don't hesitate to share collectibles with me! I have enough of them to get you one of the achievements)