As addictive as crack or a Raymond E. Feist fantasy story.
A wide range of maps, a large collection of weapons and numerous class styles with their own unique damage bonuses provides a humungous range of replayability, even if the action is essentially repetitive. Different levels of difficult allow players of any ability to join up and kick the .... out of the enemy forces, while trying to compliment their range of skills to overcome the latest challenges.
While I dont really enjoy or play many online games and very rarely those that do not have a campaign game, this took me by surprise and has already seen me notch up 330+ hours in a few weeks.
Very addictive. Surprisingly never gets stale. If you love your shooters and killing zombie targets - this one is way better than L4D imo. By a mile!
I would score this a 10/10 purely for the enjoyment and entertainment it brings. But against other games - the fact there is no multiplayer and once you've gone through each character type and gotten the gold star its does get repetitive to a large extent. Still I doubt it will leave my hard-drive and will always be there for the odd game now and again.... especially during the season achievement events!
So overall 8.5. Better than the average score given here.