If you like open world / "sand-box" / GTA style games, then you will most likely enjoy Just Cause 2. Like everyone else I have a long list of games that I am playing or want to play. Starcraft 2 is taking most of my time, but I'm also playing Red Dead Redemption and Mario Galaxy 2.
Not sure if anyone cares, but I compiled a list of games with an average rating from numerous sites (Gamespot, IGN, Metacritic, GameRankings, ect...). Here are a few from that list I'm playing:
Mario Galaxy 2: 9.9 /10 !!! (highest rating out of any game this year)
Red Dead Redemption: 9.6 /10
Modern Warfare 2: 9.4 /10
Just Cause 2: 8.4 /10
Other 8.4 /10 games: Borderlands, Bayonetta, Ninja Gaiden Sigma 2, Red Faction: Guerilla
So JC2 ended with a slightly above average rating. I would definitely pick this one up @ 50% off. There isn't much of a story that I remember, but you get to blowup anything and the vehicles keep it interesting (Merc 2 is a good comparison - only w/ better graphics).
And yes, Mercenaries 1 was better than Merc 2.