Sure, the story missions can be done fast if you are in a hurry, but those, while very well done, is not what you get the game for. No, you get the game to play around with the fantastic physics. To remove key supports of a tower and watch it tip like a hammer onto your foes, to drive mechs through bases and to place tiny black holes inside particularly large buildings and then use the resulting ruble for cover. You buy it for the sidequests, who provide you with plenty of opportunity to exploit the physics. You buy it for one of the best and most unique multiplayer experiences ever. You buy it to get acces to a party game like no other. You buy it to get the best open world game since GTA3.
And you buy it to kill people with an ostrich.
Personally, I have had more fun with this game than with both Mass Effect and GTA4. Sure, it hasn't a story worthy of even GL, but unless story is a live or die issue for you, you probably won't care.