Alright, so I got the game, played it for a few hours and this is my result...
Plays EXACTLY like the last game. The world is a little bigger, you can swim and drive boats, more factions, annoying-as-hell pirates, and WAAAAAAAY more story. I have the PS2 version so I can't do online and the graphics aren't as good as the PS3 or 360 versions. (They are good on the PS2 though.)
So, basically if you liked the first Merc game, you'll like this one too. It has a lot of good humor. (Your support jet pilot is always drunk, even when flying)
So, if I gave it a score, I'd give it about a 7 out of 10. It's a good game, they just could have done better.