Last year I ran a short campaign using Gamma World 4E. It was a lot of fun, most of the players had never played a tabletop RPG before and I hadn't run anything in over a year so it was more or less a shake-down cruise for everyone. After running through the terrible railroad that comes in the original boxed set I started writing the next adventure. Taking place in a local blown-out office building taken over by diminutive techno-fetishists in red haberdashery, I never finished the bottom floors. There's a still a lot of content and a few neutral factions to give it a more living-dungeon feel. Not so useful to anyone not running something 4E, although anyone with enough old GW stuff could probably replace the monsters with ease.
I'm afraid the whole thing petered out in the end, killed by a combination of mismatched schedules, summer travel, and malaise. It cemented in my mind the things I love least about 4E, the risk of option paralysis, no space for a character to hang out-of-combat utility, and samey nature of many of the powers. But never mind all that! It's a fun little system, it out-gonzos a lot of other games and makes for crunchy, tactically interesting combat.