Jesus Christ, this game is fantastic. A marked improvement over the first, which was already an already amazing game.
Everything has been scaled up, but it is also a much more fluid, faster-paced game. Gone are the fetch-quest like "Find named baddies, cuff/kill, gather intel to get big bad, rinse and repeat". Multiple, open-ended bounties for both people and buildings make for a good way to both cause massive amounts of destruction, and get paid for doing so.
I really doubt this game is going to budge from my disc tray. At least, until Fallout 3 rolls around.