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Its not terribly long. i dont think its terribly short either tho.
I have that problem with all FPS, i go thro them pretty quickly without much problem. i breezed thro Doom3 in just a couple days.
i certainly wasnt bored at the end of HL2. I quite enjoyed the story line. and am going to be waiting to see what happens for HL3.

and expansions? perhaps. There was rumors about being able to play as barneys point of view or Alyx's but i cant see there being any huge expansions if HL3 isnt that far off. unless they make HL3 the expansion.