I didn't agree with him much of the time, but it was sad he died of brain cancer.
If nothing else, he was a major force within the Democratic Party for many years. There were probably many who regretted renominating Jimmy Carter over Kennedy in 1980 for the Presidential race against Ronald Reagan. He was more eloquent and more forceful in his presentation, and may have mounted a stronger campaign than Carter did. (of course, that doesn't mean he would have won...)
And sometimes he did cross the aisle, as he did to pass legislation early in the George W. Bush administration on No Child Left Behind.
While not forgetting that he was controversial in many ways, one must give him credit for being a major player within his party as well as a powerful voice in the Senate in general over for five decades.