After many years of internal debate, I have decided to throw my opinion into the fray. This is a great game for those of us who would like a FPS with realistic, strategic gameplay and great graphics (just look at the beautiful particle and light scattering effects as the sun shines through the jungle foliage on an early morning attack on a timber camp) all mixed up in a wonderful open world.
Seems like a lot of folks detest this game because of the slow pacing between missions, the perpetual respawning of enemy at previously cleared checkpoints, and general lack of full-time run and kill chaos. We all know it's a game but let's try to look at it from another perspective. IMHO, I don't think it was designed to be a full-time run'n'gun type of game. Stick to COD if you like that sort of thing.
Realistically, if you single-handedly wiped out every soul at a checkpoint and had no one on your side to take it over, wouldn't the enemy just clear out their dead (disease, you know...) and re-populate once you left? I would.
I find that if you study the map, look for different routes to your current objective, like; using the bus fast travel system, then shortest path or one with strategic safehouses for your saves, you may end up walking and swimming some more but you get there and get the job done while possibly avoiding some or all of those pesky checkpoints. This illustrates some of the beauty of this game, there's more than one way to do it. Another thing I really like is put on the headphones and you can pinpoint your enemy in the jungle or tall grass by the noise they make as they are hunting for you. Look for their muzzle flash during firefights! There is an intrinsic value of realism like this that the designers bestowed upon this under-rated title.
While I truly believe nothing is ever perfect, I am giving it a 10. Remember, its just a game folks, and a darn good one at that..