MW3 offers two multiplayer modes, each with a separate progression path. Co-op is essentially a wave battle building on CoD's "Survival" zombie murdering marathon, this time with human opponents straight out of the single-player campaign. Ironically, you can play Co-op solo if you wish, and it has its own level-progression and set of unlocks. Like all arenas, you have to be of a certain achievement-driven mindset to enjoy the repetitive gameplay, and I personally enjoyed the variety offered by several of BF3's co-op missions (where two players can pilot and co-pilot an attack helicopter, for example) much more.
Traditional multiplayer mode is, in MW3, maybe a little too traditional. Most CoD dabblers will be happy to get fundamentally the same experience they got in the previous Modern Warfare games, complete with new maps and weapon, icon, and dogtag unlocks and a level progression that offers level-up perks such as sprinting longer or reloading faster. If you haven't been initiated into the Call of Duty ranks, however, expect to be owned early and often. Design flaws (and yea, I regard them as flaws) such as insta-proning, sniper rifle fast-scoping, and bunny hopping to avoid fire sap the game of its military shooter realism and propagate CoD's already closed-off community. The multiplayer side of the Modern Warfare 3 is pure twitch, and if you aren't, you shouldn't expect more that 5-6 hours of single player thrill ride from this title.
Modern Warfare 3 somehow managed to pack twice as much single player excitement into its 5 1/2 hour campaign that Battlefield 3 did in my 9 hour playthrough, but, yea, it only lasted 5.5 hours. If you enjoy wave battles and have a friend of a similar mind, you might stretch that figure a few hours with co-op. But what really decides whether MW3 was worth $US 60 of your hard-earned gaming dollars is whether or not you're truly hardcore enough to rank up in multiplayer.
If you're gaming time is limited, the promise of Call of Duty Elite seems lackluster, and if you're more drawn to other titles coming out this holiday season, it's probably best to wait until MW3 winds up in the bargain bin.
Call of Duty: Modern Warfare 3 is a solid military shooter with an enjoyable and fitting (if brief) campaign, but with co-op being little more than a time-wasting ring battle and multiplayer still overly punishing to CoD neophytes, it's hard to recommend this game to anyone other than the Modern Warfare hardcore.