Learning to surf/travel with the grappling hook and parachute made the game a blast for me.
The same for me. Except it took me like 10 hours into the game before I figured it out. I would suggest anyone new to the game to figure it out early as it is a fun and fast way to travel.
The game itself is a blast, the voice acting and "plot" are pretty terrible, but honestly that's just fine with me. Some of the missions are pretty fun, others are really predictable. The controls are easy to get used to and the auto-aim is pretty generous, but it fits the style of gameplay.
Due to the sandbox style it takes some getting used to the mission structure (that is, almost no structure) if you are used to a FPS. There is very little of the game telling you what to do and when.
There is a TON of island to explore and plenty of things to blow up and firefights to get into. There are endless ways of playing the game. Personally I find it a lot of fun to ride on top of a friendly gang's vehicle as you blow things up as he drives.
I've also bought it for $5, and so far logged in about 20 hours. It is well worth the $5.
Oh, and the game's visuals are really quite nice
Sometime I want someone to make a game like this, except with the option to have realistic driving and flying mechanics. Plug in a nice wheel and joystick and you could have some serious "simulation" fun.