Half-Life 2 has almost every aspect of a good single player game, with an added bonus of fantastic multiplayer add-ons.
Half-Life 2 is a visually impressive, and very scalable game but does have its flaws. The some times predictable, and repetitive puzzles stand out at the end of the day. Also with no memorable characters the player feels unattached, wondering just why he or she is trying to help the unfortunate people in the Half-Life universe. Finally the heavily scripted game play leads the single player experience to be good the first time through, but quickly becomes boring if played more then once.
Overall this game is a must have if you enjoyed the game play of Counter Strike, or Day of Defeat. Be cautious though of buying the game in a retail environment though, as that version of Half-Life 2 does not come with Day of Defeat: Source.
If you are buying this game just for the single player experience I would suggest skipping this one, in favor of other games with a better story.