When will Valve EVER make a game that isn't great?
So, the game only has 20 levels that run for ten minutes or less each. The game itself is short. So I certainly wouldn't recommend buying this game unless you at least have someone to play with (or, better yet, you should have Live, so that you can play with three other people). The achievements are fun little challenges that you can shoot for and they do add some decent value. Every time you play the game, it will be modified by the randomly-generated enemies, or by the AI-System called 'The Director' that can change the difficulty, the music, or the effects, just to name a few.
If your team isn't working together in the game, you aren't likely to last very long. You have to work cooperatively and strategically to ensure that you make it to the end of the campaign alive.
The original fun of the game is likely to float your boat, but buying this with the notion of playing it alone will definitely sink it.