Great links and stuff, just to add my 2 cents:
My theory is this was meant to be a DLC that was so well done (SR3: Enter the Dominatrix) they turned it into a full sequel (ala SR4). It's definitely longer and meatier than Tina's (ha!) but falls well short of being it's own full title, and the utter lack of meaningful DLC's, some of which reek of features/characters that should have been in release (Jack clone). This should have been a 39.99 game with a lot more emphasis on multiplayer. It's by far the weakest in the series in it's entirety, however, and a big one...it has the best combat, by far. It adds a third dimension game similar to something I played in the late 90s or early 00s, Tribe maybe? All the added oxygen tanks and specials means more customization, and the characters in the game are far more diverse than in previous (soldier/pistols/siren/sniper/pet) routine. Jack uses clones, Athena a shield, and both are damn fun to play. Oxygen becomes very trivial, very quickly as you learn to manage it in the first few hours. You can suffocate your enemies in multiple ways, fire, breaking their facemask, and the added ability of SLAM! turns the game into a Enders Game drift of shooting, culminating with an atomic blast of a drop on your enemies (which can also shatter their erm faceshield thingies).
I'd give it an 80/100 It's a solid B- as long as you're not paying a whole lot. But AAA it aint.