Played it at my nephews this weekend. Absolutely loving the class changes; it's so flexible while also remaining balanced. I like how you can customize your class to fit your needs at the expense of features you may not actually find beneficial to your playstyle. Don't use secondary grenades? Swap them out for another attachment on your pack. I spend way more time customizing my classes than I ever did in the other games.
I haven't familiarized myself with the maps quite enough to say whether I truly like them or not though. Some I downright despised though... always felt exposed, always someone able to shoot me from behind it seemed.
SMGs seemed downright overpowered at times, and assault rifles seemed underpowered. I didn't quite like any of the assault rifles, and it sucks they did what they did to the AK-47(well, the AK...25? I think?). I loved the AK-47, used it in all of the past CoD's... but this version seemed weak, and wasn't worth it with the recoil. Really sad about that. In a series I always used assault rifles in, and never EVER decided to try out SMG's... I found myself exclusively using SMG's in Black Ops 2 and eventually just stopped trying with assault rifles. Maybe this is the balance people were asking for, feeling assault rifles were too good to use anything else... but I dunno. I feel SMGs do mid-to-long range fairly well while dominating close range as well, and assault rifles don't do long-range well enough to justify using them over an SMG for that tiny bit of range.
Just first impressions though. I also liked the small touches, like being able to camo not only primary, but now secondary and even your combat knife. In fact, if it's a weapon you can camo it.