That's the rule set that started it all for me! Woo Hoo!
The first module, GW1 Legion of Gold was the best. It was very akin to an AD&D adventure. Unfortunately it just didn't catch on with my friends and our Gamma World campaign died out.
That cover is my all time favorite piece of gaming art. It's by Dave Trampier, and the cover to the boxed set is even better, in all it's pastel colored glory! There's a poster for it out there, that says, "Gamma World... Is Here!". I've been trying to get my hands on one for three years now - the only copy that's come up for sale recently was from Gary Gygax's personal collection, and sold for a ridiculous amount of money. But some day - SOME DAY - it shall be mine. Oh yes, it shall be mine!
I used to play Gamma World 1st Edition quiet a bit. I like that the rules system was complete enough to run a game but acted more like a skeleton that you could hang a lot of your own ideas and rules on. Great game. I look forward to playing Mutant Futures and Mutant Epoch, which both remind me of Gamma World (Mutant Futures being a direct retro clone).
The best part of GW was the GW to AD&D conversion chart in the 1e DMG... you could have a total Thundarr the Barbarian gonzo campaign!