The campaign is just awesome so far. After the first few missions it completely opens up - I'm really enjoying it. I just love how alive Chenarus feels, I'm currently playing the 'Manhattan' mission and it actually does feel like you're in a warzone. While you and your team are off completing a bunch of different objectives, there's hundreds of units from different factions populating the entire map, patrolling, fighting - all of it completely unscripted. Not to mention the civilian population caught in the middle, some of whom might tip you off about nearby enemy camps if asked, whereas others might just tell you piss off or ignore you completely.
You even get random missions and assignments along the way. Only on my second attempt at the mission did I get a call asking to secure a crash site for a downed civilian chopper, which of course you're able to accept or ignore as you see fit. There's a surprising amount of choice on the part of the player in almost every mission I've done so far.
My only real complaint would be performance, specifically in the campaign. It's like my CPU is acting as bottleneck, I get pretty much the same performance (never above 26fps) regardless of settings. This is with a Q6600, and even people with i7's are reporting the same thing. If I open up the editor and wander around I'm getting 40-50fps with the settings almost maxed out, yet during the campaign in the same spot I'll be getting 26fps with everything set as low as it can go.