Well, my opinion is biased since Cao Cao is my favorite person in that time period and somewhat of my role model
I think that he wasn't as evil as the SGYY makes him out to be, but he WAS a completely ruthless person who would stop at nothing to get his ultimate goal. Kind of like what businesspeople should be; do whatever is necessary to dominate the competition and win out in the end. Cao seemed to feel that the ends justify the means, and if you think about it he was justified in almost all cases of his "cruelties". Dong Cheng, Ma Teng and the rest wanted to kill him; would you rather let them suceed or get them before they get you? Lu Bu was a signifigant threat and needed to be dealt with. Yuan Shao was acting under the 'Girdle Decree' and would have attacked Xu Chang if not taken care of.. you get the point.
IMO the Simas were more villains than Cao Cao was (sorry to you Sima fans!) because they deliberatly betrayed the trust of the Cao family which was the ONLY reason they had any success to begin with. Cao Cao was forced into acting against Emperor Xian in some cases because he (Xian) was a weakling who IMO was probably saying to himself "I'm the Emperor but I have no power and never did since I took the throne.. Cao isn't the man Dong Zhuo was so maybe I can call him a traitor to get rid of him." (Meaning that Dong Zhuo would have killed Xian if he tried that; Cao was a lot more loyal), and thus moaned about Cao Cao being oppressive and a tyrant.
So, in conclusion and again IMO, Cao was no more of a villain than nearly anyone else in the Three Kingdoms era, he was just a lot more resourceful and knew that you had to take drastic measures to ensure peace. If those same principles were applied to the current day and age, there would be a lot less problems, I think.