Played it for a couple hours, I do believe there is going to be a lot of replay value in this.
As far as if it's linear or not. Well, let me clear that up if I can.
It's not linear.
But it's not a sandbox. There's a difference. You can have very different experiences in it. You don't have to go in the same routes, though there are bottlenecks you might have to. If you look around, there are lots of little routes to take, and with very varied enemy AI, it's going to be different every time.
I mean, it's all a matter of semantics, in that the depth and the options open to you increase the more experiences you get. There is a learning curve, and from what I understand, you won't reach the peak until after you play the game through and then pay some more. This isn't a game you play through once or twice, it's a game you might play for a long time.
In that vein, I don't think Metal Gear Online is meant to be played until you've absolutely mastered the single player. If I had to say what I felt about it right now, I doubt I'd say I had a better time than Osy playing it. But until any of us spend a long time, getting all the little details of the game down, then Metal Gear online isn't going to be designed for us.
As for the story, can't complain so far. I think.. yes, it is getting to look better than GTA4's story, but... GTA is a much better story for "mass market", in that it's essentially a bunch of mobster movies and an immigrant movie put into a blender. Halo 3 is essentially a really cliche army movie and a somewhat bad sci fi movie put into a blender together. These are all fine for video game plots. Metal Gear Solid is a bit different. It's original and mature while being bloated, full of in-references and trying to mind-fuck you at every turn.
You can't compare their plots because there's no comparison. They aren't the same species. It's like comparing 12 Monkeys to Stargate.
The only thing I really have to take issue with is saying that MGS4 is just "above par" in the graphics department. I think we can look at 8 games so far this gen and say these are the best looking games.
UT3
Uncharted
Heavenly Sword
Call of Duty 4
Mass Effect
Assassin's Creed
Gears of War
MGS4
Now you can make an argument that one is better looking than the other, and I don't think it's going to come down to much more than opinion. On top of it all, people might want to take different things into account. Like... do we look at animations, like Uncharted surely has the edge in? Do we take how much is actually on screen into account? In which Gears of War might lose points? Do we look at frame rate? I don't know.
For me, I might say COD4 and MGS4 are tied for first. Firstly, I don't like games where they make everything really shiny and put filters over everything to try and make it look better. I don't think cartoony graphics should get an edge cause it makes it easier to look good, which is why I didn't include Ratchet and Clank.
Metal Gear Solid has environment that looks like real environments. It has people that look like people. It has effects that look like their real effects, and it looks cleaner, just as detailed and smoother looking game to show for itself than say, Gears of War.