I never beat the SP either. I only played the first 3 or so levels and when it hadn't gotten any better, just moved on to the MP. I enjoyed the MP a lot, though. It isn't as much fun to run around by yourself since that will just result in getting sniped, but playing with 3 others so you have a 4 man team working together is a lot of fun.
In one match, my 4 man team held a warehouse from enemy soldiers and tanks throughout the entire round. By the end of it, the thing was as destroyed as it possibly could be. That was probably one of the most intense multiplayer matches I've ever played.