What could be said about Spelunky that hasn't been said yet? Yes, it's very hard. But yes, it's one of the most enjoyable games I've ever played. Thanks to its randomly generated levels, the replayability is almost infinite. What's absolutely delightful about Spelunky is that it's one of those "perfect" games. That means that you'll always know why you lost a game, and how you could improve. There are hundreds of tiny tips and tricks to know and discover, and the "secret" levels are always a joy to go through.
Spelunky can please the most hardcore players, but it also allows a more casual gameplay. People will either want to complete the game as fast as possible, or with the biggest amount of gold, or simply try and beat the final boss. In the end, what's the most amazing thing about this game is that you won't necessarily play it for its entended purpose (beat the game, deliver damsels, get treasures..), but rather for the journey you'll embark whenever you press Start.
Absolutely amazing game, I couldn't recommend it enough.