SP is awesome, has a decent story and some pretty decent character development. Thoroughly enjoyed it.
MP is slightly laggy with performance issues atm. Lag I can expect because of the failure of GS/ATVI to have the hardware to support the initial rush to play. That will die down soon enough. The performance issues I expect will be patched within the week.
All in all, I think that once the MP issues are sorted, I will be playing this for many, many hours.