MP was good for about a month, then everyone realized how broken it was. What was there was great, some things needed balancing, but the netcode was the biggest problem. If you were hosting it was almost impossible to not dominate. The sense of tension when you're the last man alive is unmatched. Very few MP games give you that feeling. I thought MP was great, outside of the shitty netcode.
SP is good whether you're playing through on coop or not.