Like most things, the truth is somewhere in the middle between myth and reality.
Since it's all just opinions, I think I'll believe someone like Ron Van Clief. He actually knew and trained with Bruce Lee. He also came back to fight in UFC 1 at age 51. I'd especially believe him over some nobody, keyboard warrior, on Sherdog.
http://www.sherdog.com/news/articles/Ron-Van-Clief-Where-Is-He-Now-18699
While Lee is mainly known for his many martial arts films, he was also interested in grappling and had begun integrating it into his fighting philosophy, under judo legend Gene LeBell.
“He was a mixed martial artist, he was grappling back then,” Van Clief said.
And how would he rate the legendary Lee if given the opportunity to train and compete today?
“I think if he put his mind to do it, he would've been an excellent lightweight, as good as any of the lightweight MMA guys around. He was tough, he really was tough. Most people didn’t realize that, he could take it and dish it out. It’s kind of rare in any weight division.”