I'm a long time PC gamer but fairly new to the xBox and was pretty amazed at some of the whining here.
So much effort has gone into this game it pains me to see people trying to blame it for failures in their own gaming skill, ingenuity and attention span.
It's Far Cry; you can do what you want. If you think of trying something, the designers have probably thought of it first and added bells to it. There is no spoon feeding and you'll have to develop in-game skills and tactics. If something isn't working for you, engage your brain and try something else. The game will reward you.
And all this moaning about driving and check points... give me a break! The factions aren't going to set up road blocks that can't stop one man with a car and an assault rifle are they? They haven't been designed with lazy-Joe-gamer in mind, they've been designed to maintain realism and your immersion in the game. All your missions are covert, regardless of who you are working for, so both factions see you as an enemy.
So, how do you beat them...? Start thinking tactically.
Firstly, they are just guards. They don't stray too far from their posts. If you have to go through a check point the best option is just to power through. Stopping to fight them each time is just retarded.
You'll need to learn to drive fast and you'll need to remember how certain posts are laid out (some have worse barricades or more LMG emplacements etc). You'll have to find a good route through which works for you.
Learn to drive off road. There are corners you can cut and some of the worst posts you can drive round entirely. Its amazing fun to swerve off the road and go barrelling through the trees. If you get really good you can look behind you and watch them trying to follow and crash.
Drive at night... off road... with the lights off. The guards can't see far at night and actually lie down and go to sleep. You can roll up slowly and then power through. You'll be long gone by the time they get up and run to the guns or their vehicles. With your lights on however, they'll see you approach and be ready.
Find a vehicle that works for you. The buggy is fast but seems a bit fragile and the rear mounted engine gets shot out too quick for my tastes. The cars are a better option if speed is your thing. I prefer the Jeep with the mounted gun. Set it rear facing before you drive - it will save you valuable seconds if you do need to take out anyone following you. Upgrade the reliability of any vehicles you use regularly so you can fix them quicker.
Only stop and fight as a last resort, and even then avoid getting out on foot if you can. If you have to bail (black smoke from the engine!) then run first, shoot later. Finding cover is everything. Sometimes you just can't avoid a pile up and getting all hemmed in amongst burning cars and exploding glass, so a good close quarters weapon is recommended (MAC-10 is my favourite).
However, there are plenty of hidden trails and paths scattered all over the place, particularly if you see a safe house that looks a bit off the beaten track. They're not marked on the map, so you'll have to explore a bit, but it is often much quicker to use them. Sometimes you can go by foot, car and boat and reach your mission destination in minutes without meeting a single patrol or check point. All this moaning about taking 20 minutes to get to the other side of the map is pathetic and usually unnecessary.
Well, I hope I've used one aspect of the game to show what Far Cry 2 really has to offer, and I hope I've countered some of the 'da map is way too big - wurst game ever' kind of comments.
Far Cry 2 may not be to everyone's tastes, but for me, that is a good thing.