Well it is like the MP mode but definitely different in ways. You've got a number of exclusive maps, five enemy factions, three difficulty settings, the equivalent of "skulls" to make things more interesting, and a variety of squaddies to choose from. All the MP factions and enemies make an appearance, and the new faction is called "Mirror", which is basically fighting against enemies who are all different classes utilizing skills Shepard would. IE Adepts use Throw and Warp, Soldiers use Carnage and Frag Grenades, Engys use Drones and Incinerate, Vanguards use Charge and Nova (f*** these guys), I don't remember the exact powers of Infys and Sentinels (although I can say Infys do stealth around). You think Geth stagger is bad, these guys stagger you out the asshole as every single one of them has at least one skill that unbalances you. The skulls are similar in ways to Halo skulls. You've got one that prevents you from using medi-gel in the arena, one that reduces your shielding by 80% (essentially making your shields 1-shotted), one that gives every enemy an extra shield on top of whatever they already have (which doesn't do a whole lot except for shield gate 1-shots on mooks), one that increases enemy damage, and one that removes ammo boxes. All skulls on is pretty tough but you aren't required to face any tough enemies if you want to complete all the achievements for it. Even Super Elites, the hardest difficulty, against any of the standard factions with all skulls on if you have teammates enabled is pretty doable. As Omega said you get all the squaddies who are alive from all the games at that point EXCEPT Mordin sadly. Mordin, if alive, takes the whole "faking his death" thing seriously. Sort of disappointed that the new SP-only faction CAT6 isn't in the simulator though.