Just completed this, bit late to the party as I got it in a Eidos compilation pack I bought off Steam a while back. Here's my take on it:
-Graphically the environments are impressive for a 2006 game, the foilage, sunset, water etc all looks lovely and tropical. Good job for such a big gameworld.
-Sadly this doesn't extend to the character animations, which are fairly clunky - very GTA3
-Gameplay wise I was tempted to say it is similar to GTA:SA, but that's probably being a bit generous. More like Wheelman crossed with Boiling Point: The Road to Hell.
-Aircraft are fun to fly, somehow they've made the control system a bit better than the GTA games
-Vehicle physics are a bit weird, you seem to have too much grip but then when in the air you float around like the car is made of cardboard.
-I quite like the way you can take stunt positions and jump around between vehicles etc, but I can't help but feel it is underused a little (compared to say Wheelman). They should have maybe done more of a tutorial teaching you how to use it.
-Combat is fairly lacklustre, all a bit 1990s really.
-It's a fairly blatant console port, right down to the inability to change mouse sensitivity

through to needing to use a hack to get 1680x1050 resolution.
-Good job with the skydiving.
-The side missions seem very repetitive and tiresome, I did a few at the start but then gave up and just waded through the storyline missions.
What I find somewhat surprising is the positive reviews this got, 75% on Metacritic including over 80% from both PCZ/PCG. So I guess my question is, if I'm not overly enamored wth JC, should I consider JC2 if it comes down in price, or is it just more of the same?