After a shitty first hour, then a meh second hour this game becomes extremely pro. After such a bad start I'm glad I put the time into this!
Before anyone considers buying this, I feel compelled to point out a laundry list of stuff:
The first hour of the game is spent on unskippable cutscenes and a forced stealth section. It's a really
bad introduction to the game - the characters suck, you get wubwub in your face; you get a game over if you're seen during the stealth section and you have no guns. There's no way around saying just how terrible this really is.
Once you start the second hour of gameplay, you start getting guns and stuff. The problem here is that the game doesn't explain any of it's huge RPG elements to you at all well - so to save people banging their heads with frustration,
here's a compendium of points which I consider to be essential to enjoying the game:
1. Travel to marked radio masts - doing this will display terrain / interesting features on your map. It's pretty much essential for planning your assaults.
2. HOARD loads of green leaf plants, these can be crafted into medkits. Yellow leaf plants are also good to hoard, since they let you see animals through walls / terrain etc.
3. Sell all the random junk you pick up from corpses / loot containers, otherwise your inventory will quickly fill up.
4. Check the "Crafting" tab in the game menu. See which animals you need to kill in order to make wallets / ammo pouches / holsters / syringe holders. This stuff is pretty important since the game only gives you meagre slots to begin with. Expect this to take up 90% of your gameplay time :P
As I said above, Yellow Plants let you see animals through walls / etc so using those greatly speeds this stuff up.
5. Capture enemy outposts. These are shown on your map with Red Flags, capturing these allow you to fast travel to a location & have a handy weapon stash.
6. Work on the Assassination sidemissions to quickly level up your XP (they're also kind of fun). Do scope out an enemy camp to flag up all the NPCs, the game insists you knife the base leader (who will have a thunderbolt icon above his head).
7. Work on the Animal Assassination sidemissions. These are HARD since you're usually only allowed to use a bow and arrow to complete these. On the plus side, you get some nice rewards (XP, special pelts for equipment).
For
weapons I recommend buying the
MP5;
M14 or
FAMAS;
Grenade Launcher;
Bolt-action Sniper. Then stick silencers + optics on all them for manly stealth + increased accuracy

So yeah, once you have all this stuff in order and have got used to the mechanics it is a lot of fun to play. Just be prepared for a game which is more RPG-ish than Borderlands :X All in all though it's much more fun than Far Cry 2: gunplay is massively improved; there are some of the best stealth mechanics seen outside of MGS; NPC vehicles are much less deadly (or annoying); and the introduction of dangerous animals makes the gameplay much more varied.
Animals appear marked on the HUD:
The game looks nice and has good performance, better than BF3
Rather than telling you useful things, most of the hints are pretty useless.
You actually get COLOUR in this game. Weapons also don't degrade, this is much appreciated.