As someone who has finished most MGS titles (except for AC!D 1 and 2 and MGS:PO), I consider myself to be a pretty good gamer when it comes to the series.
As for MGS4, I'm sort of struggling. There's a lot of new gadgets and techniques, the AI is a lot smarter and the environments seem a lot more interactive. I love the use of the MK II which is a lot of fun. I'm taking a little while to grasp everything, especially the controls because I've been playing my 360 non-stop lately.
The cinematic sequences are, like always, brilliant and great to watch, especially if you're familiar with the story.
I'm having trouble understanding what they've meant when they said that MGS4 is open to newbies. I can't imagine some of my friends who've never played a MGS game before being able to fall straight into this. The MGS franchise definitely isn't very accesible for new players.
Anyway, a full featured review will be up within the next 8-10 days. We aim to finish each game 100% before reviewing it and we have limitations set out by Konami and Atari on what we can discuss. I'm only 2 hours in at the moment, but just wanted to vent my own grasp on the game so far.
Prognosis? Loving every single second of it!