My review of Gamma World is up at GameCryer.com. As an introductory game I think it's superior to the D&D Red Box. While the Red Box gives you a map, some tokens, dice and a barebones version of the rules, once you get past the initial adventure you need to jump to the Essentials books.
Gamma World is much closer in spirit to the original D&D Red Box because it gets you started in the game ... and then lets you keep going by providing rules for character levels 1-10. Admittedly, it's an extremely streamlined version of D&D 4th Edition and that makes it easier to package, but still, I think they could have done something similar with the Red Box.
As for the game itself ... it's a blast! It's quick, quirky and powered by chaos, which made it a good fit for our Blackrazor playtesters. While it's 4E DNA make it unlikely my group would play it as our regular campaign, I'll be actively looking for Gamma World games at area conventions.