Part legend, part true. But I would a whole lot more true than the story of a Virgin giving birth and etc.. Etc. Until you get to the more mystical parts.
OK, I'm confused: is the story of prince Siddhartha Gautama (the Buddha) made up/legend or somewhat fact? Are parts of it true and false?
An aside, before answering.
I once heard a Taoist priest interviewed on the radio. He was asked, "Do you really believe that Lao Tsu (aka Laozi) spent 85 years in his mother's womb and was born with a long grey beard?" He said, "Of course not. But if believing such a thing helps people to live better lives then I will not tell them otherwise."
All religions need something otherworldly to hold the minds of believers. Siddartha, Jesus, Abraham, Moses, and so on, all had followers who claimed supernatural gifts for them.
That doesn't mean, of course, that Buddhism, or any other religion is worthless. It's the way religion is presented and exploited by professionals at all levels that devalues it.
As others have suggested, the answer depends on what you mean by 'true.' According to legend, for example, when Queen Maya gave birth to the baby Buddha, he was born from her rib section rather than the usual route...the symbolism of being born from her heart.
Then she died a few days later, but would still come down from heaven when her son needed her for advice.
* * *
After the Buddha's death, there arose great controversy as to which of all the sayings ascribed to him were really his. This quandary continued until the eighth century A.D., which is about twelve centuries after the Buddha lived.
Finally the wonderful monk Shantideva came up with the answer; that if a saying led toward happiness and away from unhappiness; if the saying led you toward freedom and away from suffering, then it was a true saying of the Buddha.
The point being, we are actually going here into another kind of definition of what TRUE means.