I've now completed it on Veteran & as an experiment, I signed up to XBox Live Gold for a free month (courtesy of Microsoft when they replaced my 360 due to the 3 red rings of death fail)
What it has opened my eyes to is multiplayer - it is seriously addictive though! Some people do go straight to the multiplayer & ignore the single player/co-op - a mistake in my opinion. However, the strength of this game is that Infinity Ward don't miss a trick - every aspect of the game is brilliant, including the multiplayer.
It plays like a shooter, but also offers leveling up system just like an rpg. So you start off as a complete Level 1 n00b with iron sights & everything basic. The more you play it, the more guns, sights & perks you unlock. This is where some of those near the top (Level 70) have such an advantage - they have silencers, can drop live grenades, are more impervious to explosions, etc. & you've just got a sub-machine gun & a pistol.
The challenge is very high & you will be respawning...a lot. But there is such a satisfaction in being able to get a good kill-streak & leveling up that the death-frustration is only marginal. Currently I'm at the not wholly impractical level 21, which has unlocked the 'Bling' perk - 2 attachments for guns - such a win.
There's lots of customizing opportunities & you can fine tune how your character behaves - do they need to be swift & silent - then activate those perks. Do you need to be like an unstoppable Juggernaut - then get those perks.
The leveling up of weapons & perks happens through challenges - like get a certain amount of kills or headshots, or scoped kills or shooting from the hip.
You will learn to love the sound of leveling up your character - just like that magical sound of unlocking achievements - win!
The maps are all in very different locations from within the game & near the game & are all well costumed to accommodate the different game types. Particular favorites are High Rise, Afghan, Terminal, Invasion, Skidrow & the very tiny Rust (what? I like small maps - more intense!) There's 16 maps in total, but I'm sure more will be added with forthcoming DLC.
The different match types are fairly varied & offer different challenges, depending on what you prefer. Are you a Lone Wolf - just wanting to find & kill anything that moves - Free-for-All. Like to work as a team for different objectives, Deathmatch, HQ, Demolition, etc. is for you!
You also represent different factions, such as the Navy SEALS, the SAS, etc. But also militia the Russians & shadow company, etc. to balance it off. It's always a good v bad forces theory, but still good to be either. Seeing as my nationality dictates it - the SAS is my favorite!
Once you get into the game, get to know the levels a little you will notice a patter of where the opponents are hiding out/trying to capture & it's then a raging war to capture the spots back!
Also the Spec Ops is ticking along nicely - I've got most of the 69 stars (59 at last count) & have even done one of the Echo levels on Veteran - epic!
Finally X-Play even rates this game best shooter & multiplayer in their annual Game of The year Show. So if you like shooters - you need to own this game - then it can own you!
PPS check out the CoD6 Wiki
