So, I've bought this one recently, following on advice from someone who already had it and played the original AvP (of which we both were avid fans).
I've got to say I'm really liking it so far. I agree with Fedorov on the SP campaigns. Both the Alien and the Predator campaigns are fairly easy on Normal - as long as you keep your kills clean and quiet, because then it's always up to you to dictate the terms of the encounter.
However, once you up the ante with Hard difficulty level, things get up to par with the difficulty of the Marine campaign. I, for instance, found the Aliens very tough to kill as Predator and it requires really good close combat skills to beat 4-5 without dying (meaning you have to be really adept at blocking at the right moment, using your heavy attack to break the enemy's blocks, etc).
As for the MP. Seval points come to mind:
a) A small number of maps, all of which are relatively small. Different maps favour different species and different playing styles. Lack of modding tools and the explicit ban on modding activities will be frustrating to those who won't follow up this purchase with additional DLC purchases.
b) The balance. I don't know, for me - coming from the original AvP and having played next to nothing of the AvP2 - the balance's good. It's not perfect balance, and I wouldn't really view this game as a good competitive game, but more of a fun game where you have to accept that you're sometimes you're the ****er and sometimes the ****ee.
The balance, as I'm viewing it now, is something similar to: Marine trumps Alien, Predator trumps Marine (if done in a stealthy way), Alien trumps Predator (if stealthy - and with its speed it's semi-manageable) and so on.
I wouldn't agree that the Predator's disc is overpowered - you have to hit, the enemy can hide behind walls before the disc gets to them, the targeting beam betrays your location.
In general the Marine is the easiest to pick up on, as you just jump in and shoot things. You don't usually get clean kills as is possible with the other two species.
With the Predator and the Alien - if they're played right - the victim has little only a slim chance of survival. I understand that that's what frustrates people, prompting them to whine - but nobody said it's an even fight.
c) The game modes. I'd say all of them have a well-deserved place and all offer good fun. Infestation and Predator Hunt are a form of game of tag (with respectively Alien and the Predator being the person tagging), albeit it requires a fair deal of discipline during Infestation. I haven't seen a single Marine victory and I've played several rounds - worse is the fact that some people don't have the patience, or don't understand the object of the game, and go off solo to get themselves killed (because you get more points for Alien kills than for pure waiting).
d) The players. All in all, flowing from the two previous points, I fear that a large part of the audience is going to be people at whom the game was not really targeted. I feel this game requires patience, because sometimes you just sit tight and hope you're well set up - same goes for the other side, when you're slowly creeping up and biding your time to strike when you see a weak point.
With no dedicated servers, leaving hosts, immature players and the like are going to be a problem, but so far they haven't been that frequent in my experience.
You will also have people whining about lack of balance or criticising you for spamming overpowered weapons ("stop spamming the disc and do something else", for instance). Again, I'd say there's no reason to whine - it's about fun, sometimes you win, sometimes you lose, and there are counters to these so-called "op" tactics.
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All in all, if you take into account the above things and still seem interested, I encourage you. You only have to remember about the potentially small longevity of the title, unless the situation with the mod tools changes (or the DLCs are going to be free).
If you feel like MPing with me, then that's "AThousandD" on Steam. Cheers.