I recently downloaded this game for 1600 MS and to be honest it was worth the money - it has certainly added new life to the game and also extra multiplayer modes.
You play as a chap named Johnny Klebitz - the second in command of a notorious biker gang known as The Lost. When the leader is let out of jail however all hell breaks loose as a truce between another biker gang The Angels of Death is broken.
The story actually intertwines with Niko Bellic's - the first time you meet Johnny in GTAIV is overlooking a drug deal for Elizabeta - although you probably wouldn't have remembered this until you got a few missions in - or maybe when one of your gang members mentions that a serb guy killed one of your friends as he was hired by a girls dad cos they were getting it on.
Anyway without ruining too much of the story line there are extras such as weapons, vehicles and map locations - the game actually makes good use of those places you may have failed to explore whilst playing GTAIV - you'll be all like OMG I didn't even know this place was here!. There are also new activities such as arm wrestling and gambling - all of which you are free to do at your leisure and not on the demand of others - stupid phone calls!
Another thing is some (not all) of the radio stations have different playlists and also the TV channels have extra programmes on - including a follow on from the hilariously funny Republican Space Rangers.
And I almost forgot - you actually see a penis not far into a game - it belongs to a guy called Mr Stubbs and although its a penis and I dont swing that way - I was like Oh Em Gee - it has officially made gaming history by being the first mainstream computer game to include genitalia. I wonder how Saints Row developers will respond to this!