As a non-Penn State alum (though I had been a fan of both Penn State and Paterno before this whole scandal, and hope James Franklin makes the program competitive again), McQueary comes off as having some screw loose.
His original story was that he saw Sandunsky showing with a boy with "possible" sexual sounds, but that he hadn't actually seen anything. Paterno says McQueary saw "fondling" while others report McQueary saw "horseplaying."
10 years later he changes his story and says he saw full-on rape and that that is what he reported.
He also was caught lying about stopping the incident and reporting it to the police. And he sued the university when the new coach after Paterno didn't hire him. Really, would anyone want someone that that kind of baggage coaching in the post-Paterno era at Penn State?
The inconsistencies shouldn't stop his experience from being used as one basis for finding the truth, but one should weigh what he is saying against his credibility.
Let's be honest, the guy was likely under immense pressure from all sides with regards to his story. Either way there is no doubt that he saw something sexual happening.