I'm giving this a 5 even though it deserves a 4 because of all the bugs, but I know with it being a old game I don't expect them to invest in it as much. There is too many negative reviews. I don't understand why. I heard there was lack of content in the beginning. I know some people simply just prefer sledgehammer style gaming.
I've played zombies and multi-player. I didn't really enjoy multi-player until I started playing this game. I like that it offers modes you don't find on mw2. You can't find knife fight, sticks and stones and you can't get prop hunt. I like that the unique party modes and that the maps are small. It can seem repetitive with them being small, but it's great for new players especially and you don't take forever to find other players and harder for them to camp which is nice. It's also nice you can level up in zombies and multi-player and what you unlock crosses over. You can also play traditional cod modes that all of them have. It also shows health in the kill cam, so you can see how close you were to killing them. The movement is also a lot faster in mw2 and tbe sliding is a lot more slick which I personally liked.
Zombies also has a variety of different modes that are fun to change it up. Also all the xp crosses over to multi-player encouraging you to play it or use different weapons you might not normally use making it less of a grind in multi-player to level up when you first start out. Also all thr round based zombies maps that they offer are not an extra cost which is also really nice.