I'm only about half way through it (I think, don't know how long it lasts) but I think it's great. I like the mix of stealth and action and the fact you can seemingly play either extreme or somewhere inbetween. It has cool challenges that help unlock bonuses on weapons too.
Plus, I'm fairly sure it is Michael Ironside doing the voice of the main character. I love Michael Ironside. (I assume he has been Sam for a while but I've never played a SC game before).
So I've really enjoyed what I've played so far. I look forward to replaying some bits to try and do a level without being see at all and the like. Plus it has a decent co-op campaign that I hear is totally outwith the main story. And it has all kinds of bonus game modes that I have yet to even look at. Pretty cool all in all.
So, I haven't played the old games, and I haven't finished this game. But I think it is pretty damned good.
Backlog: Fable 3, Velvet Assassin, Halo Reach, Blue Dragon, Sacred 2, Risen, Mind Jack, SC:Conviction, X3