OK, I bought the game for the SP, and it is good. But the MP is some of the best I have played in a long time, but I can see how it would not be for everyone. It is not a type of MP for the run and gun brainless FPS's out there (not insulting you, but the level of grey matter required to play those is pretty low).
The MP is a game of stealth and strategy, more so than any other MP experience I have had. It is also a game of patience. It is a point based system (i.e. highest points wins the game) and I have spent 2-3 minutes sometimes tracking my target silently to get 1000+ points for the perfect kill, while the running morons on the map have gotten 4 or 5 kills for 100 points each. I am frequently in the top three and have the least amount of kills.
I welcome this type of gameplay. It really makes you feel like you either the hunted or the hunter (many times both).
To describe how the MP actually works (if it is helpful to you). The standard game is a free for all match. Every player has a unique model. The map is full of NPCs that look exactly like each of the players' models. Each player is given a target to assassinate and a radar shows you the general direction of your target, but it is vague. Your target is the only person you can kill (besides innocent NPCs). So your radar may lead you to an area that has 5 people that look like your target. You need to either study their movement, or see if one leaves and your radar changes, but you won't know which is which. You will also have people hunting you. So you have to keep your eye out for someone you think might be an assassin to kill you. If they are smart, you will never see them coming. And you need to know when to stay hidden and when to attack.
There are skills you earn through experince points that help you blend into the croud and hide, help you kill from a distance, help you escape an assassin and help you assassinate silently.
It is a ton of fun, especially with friends. I love telling a friend, "hey, look up" as they see me diving at them to stick a blade in their eye socket. Or as I say "do you feel alright", as I poisoned them 5 seconds ago and they slowly die.