If he hadn't been caught cheating on multiple occasions this would be a slam dunk answer, great coach. However, since that has happened multiple times in multiple areas of the game across spans of many years it requires that the rest of his record be called into question. There are plenty of other, well discussed, situation where New England chicanery has been suspected yet unproven. If the coach had maintained a sterling record it would sound like sour grapes to mention them. I liken this to our older fans griping about Don Shula's participation on the competition committee and his influence on officials. Don Shula has an impeccable record with multiple teams over decades. He was great. You can dislike him but he was great. Belichick on the other hand, has been caught actually doing the things people have suspected him of. There is no accomplishment on his record which you can't call into question as a result.
He's not the only person in sports this has happened to and he won't be the last. We said goodbye to one of them today in Alex Rodriguez. The man hit almost 700 home runs and will go into retirement with barely a ripple of attention because he got caught cheating multiple times. I personally believe the Cal Ripken was dirty as hell during his games played streak but he was never caught and nothing will be proven so I will accede to his greatness.
Shula may have been a slimeball but Belichick is proven to be one.