Now that Blizzard has announced that it is developing Starcraft 2, this is a great time to go back and pick up this classic real-time strategy game.  It's pretty much accepted amongst serious RTS enthusiasts that Starcraft is the gold standard of the genre.  As many other reviewers have mentioned, this game is polished, incredibly well balanced, and a lot of fun to play.  Both the single-player and multi-player features are enjoyable (pretty amazing that you can still get games on battle.net, considering that this is product is ten years old), and Blizzard still provides patch support for the game.  The included strategy guides are rather small, unfortunately, and are difficult to read, but they aren't particularly good anyway, so it's no big loss to not read them.