I'd give it a 7.5, experienced no crashes or frame rate drops at all though. (On an i5 + GTX970)
If you enjoyed L4D and similar, you'll enjoy this. More so if you have people you know to play with. The first time seeing a practically literal wave of zombies rolling towards you makes for some interesting reactions!
Have missed this type of co-op for a while, been having a blast with friends. Upping the difficulty makes things more interesting for sure. I cleared the campaign quite quickly, wouldn't recommend rushing it or you will get bored quicker, naturally. There are only so many chapters at this early point. But with harder difficulty and punishing friendly fire damage, you have to think and play better. Good satisfaction getting through a hard part by working out a plan with your team.
The multiplayer has a lot more classes than the campaign, and PvPvZ is interesting to manage and play around. There have the traditional modes, but with the twists of zombies and hordes interrupting the battle.
Definitely give it a go if you enjoy zombie games and surviving with friends.