I love the multiplayer on Mass Effect 3...it's a shame that it isn't free for used game buyers, because it's pretty amazing. Everything that I love about online multi is in there: leveling up your multi character, different classes / powers to choose from, every kill gives you credits to trade in for random new weapons, and three different difficulties.
There is only one mode of play, a 10-wave 'Horde Mode' where different enemies come at your squad in 6-7 different maps with increasing difficulty. At first I thought this would be a turn-off (I really like variety in online matches) but working with three random teammates is just so damn satisfying that it offsets the monotonous nature of the game. There are some random "protect this spot" and "kill a certain enemy" missions that pop up every game, which has the team running to different points on the map.
After sinking 20 hours into multi, it's hard to get into the single-player battles of Story Mode where your squaddies are AI-controlled with only a couple control options. I've been in multi battles with straight-up idiots who spread out and die fast, and I've been victorious over Silver-level multi missions with 3 pros with killer tactics. The variety, as well as the option to invite friends into your party, makes multiplayer so awesome! My AI-controlled teammates in single player feel like lifeless robots in comparison.