I've been playing for about 3 hours, so here's some early impressions:
*Game does have massive pop in, but this is similar to Gears 1 and Gears 2 in that it happens before the actual battles take place. Once in game it was hard to spot(360) pop in since I was constantly on the move.
* Offline bots even on hard are pretty retarded. They won't give the normal gamer much of a challenge in anyway, at least no in the first 2 missions.
* Challenge mode is a cool idea. Do things that help you learn the game while unlocking tons of shit. Good move by Splash Damage.
* Online was lag free for me the first 2 games I played. We couldn't win as the Resistance trying to escort this guy in mission 2 across the map unfortunately.
* The flow of the game is fanfuckingtastic. Shit is always going on, there's chatter coming in your headset and off your TV, stuff is blowing up, people are jumping and hopping. One of my favorite parts thus far is there's no k/d status checking anywhere from within the game, so you are totally and 100% encouraged to just have fun and work together. It says you can go online to see that sort of thing and put in some code on their main website, but I wasn't able to find where on the site to do so. If anyone knows let me know.
* First hour felt a bit WTF, in just adjusting yourself to how SMART works, weapons, movement, controls, etc. I also think it's a shame you don't start off with the light and heavy body choices, but it's a small complaint. After that first hour the game feels to open up more and more, and taking it online you see just the sheer amount of insane fun that can be had.
*Character customization without a doubt is my favorite part of the game so far. You can definitely get lost in tinkering around with things you unlock.
So overall I'm throughly enjoying myself as first impression. I feel this game is more team fun focused than games like Bad Company and Team Fortress try to be, and filling your role isn't something you have to think about at all, you just naturally know what to do thanks to prompts within the game as quick reminders to keep you playing as your class.
People on the fence for a day one right now I'd tell you go out and buy it based off of my experience, but keep glued here to Gaf to see what others say to get a bevy of voices to let you make the right call.