If you get the chance, replay a campaign mission, just to see how much the smart spawning system changes things.
So I died in the middle of declassified missions a couple of times and the enemy spawns noticeably changed on my next attempt both times it happened. One mission started with a lot of Wretches and some Boomers, when I failed it and tried again there were less Wretches and more regular locust. Later I had the same situation pop up with a different mission and found myself dealing with a lot of blade-wielding Boomers, I died and on the next attempt those blade-wielding Boomers had been replaced by the Boomers with Shields and explosive flails (or whatever you'd call them). The whole declassified mission thing is neat, too, the latter mission I mentioned had the room filled with dust that you could barely see anything through, with Wretches and blade-wielding Therons running up on you, it was a lot of fun.
So far I'm really enjoying the game, Gears of War is still a lot of fun. I'm really enjoying the way declassified missions change things up and the occasional portions where you have to hold down a room or defend something from two or three waves of Locust are also a welcome addition. The way it gives you a score at the end of every checkpoint maybe breaks up the flow a bit, though.
The story and dialogue are largely uninteresting so far, hope it picks up in that regard, but it's not the end of the world if it doesn't.