It's hard not to lapse into a copious amount of superlatives when reviewing Fallout. The gameplay, the amount of options, replay opportunities, humor, plot, and design are all 1st rate. Astoundingly good for its time, and still great fun to play. For some reason, newer games just don't get me as involved or invested in playing them. The necessities of game creation in the years before quad-core processors and 1-gig graphics cards included a huge focus on gameplay, style, and polish that is almost never seen now.
The game is a brilliantly sarcastic mix of the idealistic materialism and shortsightedness of the American 50's, near-complete nuclear holocaust, mutants, radiation, and a world that takes on a life of its own at times. I still laugh and marvel at things I did in game back in the day.
There's much more to say about how incredible the game is, and how immersive and inspiring it can be, but it'd be redundant. Just accept that it is incredible.
The compatibility is great as well. Getting the original version of the game to work with XP and Vista is a challenge; I needed a bunch of 3rd party mods and edits to make it work on my system, I could never figure out how to get sound. I was pleasantly surprised when I had the GOG version up and running in minutes (excluding the download, which was about 3 hours, 1 meg/second).
Fallout is incredible. You now have no excuse not to live a survivalist's life in the deserts of the future; all it costs you is six bucks and 600 megabytes on your harddrive, and you have everything to gain.