For what it's worth, I should add that this is the third version of Gamma World that I've played, and it's the fourth version I've owned (fifth, if you count mutant future's PDF). In addition to this version, I've played Omega World (a d20 Polyhedron minigame using 3.0e rules) and Gamma World 4th Edition (which came out a year or two before third edition).
And honestly, this game is my least favourite of the three in some ways, and most favourite in others. I like it less because it lacks hindrance mutations as a permanent part of your character (it's fun rolling an awesome four-armed level-draining furiour badger only to find out he has leukemia!) and because the entire game is aimed towards short campaigns - I really don't think you can turn Gamma World 4e into a campaign without a LOT of work.
And I like it more because of that exact reason. It identifies a goal for itself, and pursues that goal. It is an RPG that plays like a board game, and dammit, I love it for that.
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Also, the more I look at this game, the more I realize it might be kind of hard to port D&D monsters into Gamma World, mostly because the damage values are definitely off in comparison to Gamma World PCs, and the expectation with most D&D monsters is that they're going against a balanced party, whereas in Gamma World, there ain't no such thing.