It took me 7 hours to beat the game (single player) at most. That is too short for a $50 game any way you slice it. Even if Co-op takes double the time it is still a little too short IMO.
really, 8 hrs is the average for all games that is not strategy games, many shooters is like 6 hrs.
Not any of the games I have bought recently with the exception of maybe Bad Company 2, but no one buys that game for single player anyways.
I had way way more than 8 hours into Just Cause 2 or GTA IV. 8 hours is short, I am sure I could quote this very forum as saying that many times.
It really depends on what you're expecting.
The single-player campaign is ~7 hours long. From what I've heard, the co-op campaign doubles that, so we'll say ~15 hours.
Compare to, say, Portal 1, which is ~2 hours long,
and currently retails for $10. Given that it was originally 1 of 5 games in the $50 Orange Box, $10 is a perfectly reasonable number for a launch price, too. This gives Portal 1 a value of roughly one hour per $5, and Portal 2 a value of roughly one hour per $3.33 - in other words, Portal 2 actually gives you 50%
more value than the original, factoring in
only play-time.
If we
also consider that half of Portal 2's content is online co-op, I'd say that Portal 2 is most definitely not a 'bad deal'.
More importantly, comparing a fine-tuned experience like Portal 2 to a semi-scattered sandbox like Just Cause 2 or GTA IV is silly. Sure, JC2 took me longer to beat (I've got something like 60-odd hours logged on it), but half of that time was travel time, and a lot of it was pretty generic and repetitive - fun, but a different kind of fun. In comparison, Portal 2 is an incredibly finely-tuned experience. A simple "cost per hour" balance to non-puzzle games is ridiculous; the pacing and design of a story-driven game simply cannot be compared to the expansive, freeform time-sink that is a sandbox game.