The short answer is this: not one whit. OK, Mel did have a pretty fair Scottish accent (in my untutored opinion), WW did lead a rebellion against the English, and suffered a gruesome death. Beyond that, however, it's pretty slim pickings. Here's a quick Fact vs. Fiction:
- Fiction: WW was always called the "brave heart" by his people. Fact: Strangely enough, this was actually what they called Robert the Bruce!
- Fiction: WW was a poor man who secretly married his one true love early in life. Fact: He was actually a landowner and minor knight. As far as we know, he never married due to his life as an outlaw on the run.
- Fiction: WW was responsible for single-handedly inventing the spearhead to use against English cavalry. Fact: The Scots had used spears since ancient times, it was nothing new.
- Fiction: WW knew Princess Isabella on quite intimate terms. Fact: Isabella was about 10 years old when the courageous Scotsman died, and they never met.
- Fiction: Primae noctis (law of the first night) was a Medieval practice allowing a lord to take the virginity of his serfs' maiden daughters. Fact: There is no solid, historical evidence for this practice.
- Fiction: Every good 13th century Scotsman wore a belted plaid, or at least a kilt-like thing. Fact: This is a complete wardrobe-anachronism. The first clear reference to such a piece of clothing is from the late 16th century.
- Fiction: Robert the Bruce (the slimy toad) took the side of the English in the Battle of Falkirk. Fact: He did no such thing! He did change up his alliances a bit from time to time, but was never an outright traitor on the field of battle.
- Fiction: At the Battle of Falkirk WW went against the wishes of the other commanders, and the Irish bravely switched sides from the English to the Scottish. Fact: As far as we know, neither of these things happened. WW might have actually been against going into battle and was pressured into the move.
- Fiction: In a lovely twist of irony, King Edward I died just as WW screamed his defiant last words, "FREEDOM!" Fact: Unfortunately, there was nothing quite so dramatic. The King of England actually survived his arch-enemy by a couple of years, and we have no idea what WW's real last words were.
To sum it all up, Braveheart was #2 on the Times' Top 10 Most Historically Inaccurate Movies list.