I just used Impulse, DLed in like 15 minutes (its only 600mb).
As for the game itself... Not all that much different, but if you don't yet have the original, buy it. Or if you like multiplayer battles (not my cup o' tea). More than anything, its kinda of a bunch of small things that add up to make it feel quite different. The melee weapons have a much different feel to them; their trajectories are more rounded off now than the ones from vanilla which fellt like you were essentially swinging in a line. There are some decent quests, including an optional start quest, although most I came across were the old typical courier missions. More armor, and generally more items overall, including a bunch of new trade goods. If you have the original, I would wait until it goes down to get it if you aren't like me and feel guilty for paying only $7.50 for the vanilla which gave hundreds of hours of gameplay.
And now for the obligatory warning about release bugs: there have been a couple bugs I came across, one involving lighting effects, another involving some script error which occured while traveling (I do not know what that bug did, if anything, just showed up in the events pane).