Once you move out of the starting area the world is near enough fully open. I loved the game. First time I tried it I didn't give it a chance, but 3-4 hours in on my second try I was hooked.
I deviated from the core storyline all the time, exploring, taking down military bases etc. I haven't played a game which made me hesitant to enter various areas due to the building atmosphere until stalker came alone.
Isn't every last base in the game part of the main story line anyway, except a couple of the tiny camps you don't "have" to destroy but have like 5 people in anyway.
Side missions and faction quests have no point at all, haven't played it patched, finished before there were any. Invincible, or as good as 2 hours in(if that) make it a touch ridiculous, 2 great fights in the game, very early on, incredibly dumb AI, ridiculously accurate weapons and powerful past the first hour of the game. Once you get a gun that actually puts bullets at the crosshair , which is basically the 3rd/4th gun onwards its just too easy. The "open world" also takes about 3 minutes, if that to get from one side all the way to the other, so open world, generally yes, tiny world, also yes.
I've said it before and I'll say it again, both this and Bioshock started off as RPG's where the general idea is harder and harder enemies but you "level up" and increase stats and get better weapons to compensate. Unfortunately both games forgot about the harder enemies, so you keep getting better weapons, suits and powers, and the enemies simply can't do a thing about you after an hour or two into either game. If you want a FPS/RPG as both games aimed for, the whole game has to scale and not just the players character.
The main bases for the biggest missions are very nicely put together, although mostly VERY short, it feels like a few normal levels of say HL2, with long runny bits inbetween getting to those levels to extend the game to me, nothing more nothing less. The atmosphere is nice but theres little to do.