I have been playing the game and an currently on mission 3. I'm having a lot of fun with it so far; not sure why they chose the part they did for the demo, because it seems like the lamest part of the game so far.
I've played most of the Metal Gear Solid games, though I'm not much of a fan of the series. Don't expect MGS when going into this of course, but it retains a lot of the MGS style humor and some other things; cutscenes are definitly more Platinum than Kojima though. The handy cardboard box is back and you can sneak through areas without killing enemies (there is an achievement/trophy for no-kill battles as well), though I haven't bothered to try that yet.
If you have questions, ask away and I'll do my best to answer them.