The short of it: Everyone had a good time. The biggest 4E-hater at the table praised the "smoothness" of the system. In fact, we're taking off our next Pathfinder session to play another of Gamma World.
I took about 2 hours to prepare for 4 combat encounters and 2 skill challenges as well as creating 6 pregens with the "Interactive Character Sheet" (also including creating power cards from scratch for each character). Also included with the prep time was pre-drawing battlemaps on grid paper, coloring in terrain with crayons, and finding appropriate minis and tokens for the characters and monsters.
While the monsters in the GW book were a little lacking, I easily "re-skinned" monsters from the 4E D&D Monster Vault. (I made kobolds "cephalopod humanoids" which really freaked out the players more than "just plain old kobolds.") I re-skinned GW monsters to make them more appropriate to the adventure (for example, I changed porkers into "Nautilus men").
The group, coming from a background in 3.5/PF, didn't seem to bite the hook for starting skill challenges, though I tried to insert them into the adventure transparently. After the session, when they complained that the game was a little too "combat-heavy," I explained that at least one of the fights was avoidable with a skill challenge.
They loved using Omega Tech and getting new cards. They seemed to enjoy the Alpha Mutations - except for the one player who experienced Alpha Flux four times in one session. Using the cards "added versatility to the characters and a fun, random element to the game," quoting some 4E-haters.
Interestingly, upon realizing that the game was built on the 4E engine, they suddenly have a lot of questions about 4E and how it's changed since they last experienced it as a group (Keep on the Shadowfell). They were surprised how streamlined and fast combats went compared to the older 4E slugfests.
I would have to give Gamma World a big thumbs up. Next session I will attempt to run the module in the boxed set to see how well that goes.
Retreater