I put 4 solid hours into it once I brought it home earlier today. I'm taking a break now and will probably continue after I've had some rest and something to eat.
I'm just starting the second act, but mainly I need to try to get used to the sneaking and my options in combat because getting spotted means some problems for quite a while after (Solid Normal difficulty).
I'm probably going hook it up through the 5.1 surround sound when I play it again so the action will be even more intense.
*edit*
The mission briefs are pretty sweet with how you can watch the cut-scene normally, view different areas or even control the Metal Gear Mk 2 at the same time the cut-scene is happening. Also, the tips that pop up while the game is installing data is kind of humurous (they range from warnings about not playing too long to warning about the danger of smoking).
