MGS4 -9 hours of cinematics, 21 hours gameplay=30 hour game. Good or bad?

[ckmlb]

Words Of Wisdom said:

If true, I'm more disappointed at the overall total... 30 hours.  For the amount of BS I've heard about how much "gameplay" this game is going to have, that's pathetic. 

So length of the game determines gameplay quality? gotcha. I guess Portal is one of the worst games I've played recently.