Its not a game to be missed for sure. IF you havent played any of the MGS's it will be pretty pap. If you understand the whole story behind MGS but never played then you will be fine.
However.... the gameplay is no way near as long as 1 or 2....
It took me 14 hours to complete 2... and 15 for MGS1.
It took me 22hrs ish for MGS4 first time, as a guide speed runs, the fastest time you can do the game, is 1Hr 22 for MGS1, 1HR 38 for for MGS2, 1HR 40 for MGS3 and 2hr 30 for MGS4. I know that speed runs dont give a true representation of gameplay length as they are done with special features enabled, but id be hard pressed to finish mgs4 in anywhere near as fast as i can mgs1. As i have said them replacing codecs with cut scenes are a good thing to me but they do make you feel that you have come out of gameplay if you see what i mean.