Lu Bu was by all accounts a great archer and horseman-hence the nickname "Flying General." He was originally merely an underling of Ding Yuan, not his adopted son, who he killed before joining Dong Zhuo as his adopted son. Unlike in RotK, there was no "Diaochan" who seduced Lu Bu-rather, Wang Yun, the man who in RotK was Diaochan's father, convinced him to betray and kill Dong Zhuo. He was also then kicked out by other warlords, and eventually ended up swapping back and forth between allying and betraying Liu Bei, Cao Cao, and Yuan Shu. The Sanguozhi (Three Kingdoms History) does testify to the archery incident where Lu Bu shot the spear with his bow. So while he wasn't the one man army that he is commonly portrayed as like in RotK, he was quite the archer.

RotK, while a decent read, basically amounts to Three Kingdoms fanfiction. It was written during the Ming Dynasty, where a revitalization of Han Dynasty ideas and concepts was happening. As such, it is hardly surprising that RotK glorifies all of the "Shu-Han" characters as heroes while denigrating those of Wu and especially Wei. Cao Cao most notably gets an extremely bad rap as he is portrayed as being a devious scheming coward that the author begrudgingly notes was competent and had good retainers. In actuality, the Sanguozhi notes Cao Cao as being "the Great Progenitor" who cared for the common people and was primarily interested in restoring order, while having a ruthless realpolitik streak to him. Liu Bei, far from being portrayed as the hero who fought against overwhelming odds and disastrous fate, is instead ascribed a behavior similar to that of Lu Bu: a brave man with grand ambitions, but was personally a bit of an idiot. And please don't get me started on the historical porn that is RotK's portray of Zhuge Liang.

We're quite sure he existed. While RotK is not a reliable historical source (it embellishes and invents a number of events, and generally makes things much more soap-opera-y), he is referenced in a number of historical works about the period.

The better known pieces with what is generally reliable biographical information are Sanguozhi (三国志, here), Yingxiongji (英雄记, text here), and Houhanshu (后汉书, here).

These sources vary on some details, and there are other sources out there (I haven't read them, though, but I am told that they don't differ from the above much, and probably influenced the writing of the later ones, such as Houhanshu), but we generally know about his life and death as a general.

He was indeed known for his horsemanship and archery, he did have a horse called 赤兔 (Red Hare), and was known was the Flying General (飞将). We know he was born in what is now Baotou, Inner Mongolia. He served under Ding Yuan, whom he killed when Dong Zhuo entered the city. He later betrayed Dong Zhuo, killing him as well. Diao Chan is fictional, though he is portrayed as having illicit relations with a maid of Dong Zhuo's.

From here, his story follows roughly what is well known- a sordid tale of swapping alliances, abandoning his wife, reneging on promises, and his eventual capture and execution at Xiapi, at the hands of Cao Cao, on Feb. 7 199. We know about his presence in certain battles, his attempted alliances, and most of his career as a general. His character is exaggerated in fiction, though apparently he was indeed famed for poor temperament and judgment. For example, his romp with Dong Zhuo's maid was not a one off: from the end of his section in Yingxiongji: 布谓太祖曰：“布待诸将厚也，诸将临急皆叛布耳。”太祖曰：“卿背妻，爱诸将妇，何以为厚？”布默然。" Note that this was written after the Wei had won, so Cao Cao is referred to as 太祖, as he is the founder of the new dynasty, though he never took the title while living. Anyway, the gist is that Lv Bu is complaining that his generals betrayed him when he needed them most, despite treating them well. Cao Cao shuts him down by responding that Lv Bu not only betrayed his own wife, he was overly friendly with the wives of his officers. Lv Bu knows that he's owned, so he shuts up (if you want a more precise translation, I can ask my girlfriend when she wakes up).

However, many oral exchanges may be fictionalized, and often served political purposes, depending on the time they were written. We have no physical descriptions of him. The things we can be more certain of are: his existence, birth, and death. His career, and presence and role in certain battles. His shifting alliances, and temperament/weaknesses. His reputation as a gifted warrior, who lacked in other areas.

Sanguoyanyi is based on fact (roughly), but incorporates a ton of fiction. It's a novel, intended to be literature. However, many of the major events, including some of those surrounding Lv Bu's life, are factual, while others are fiction, such as Diao Chan, or the 3v1 at Hulaomen. Hulaomen, for example, isn't really mentioned in Yingxiongji or Sanguozhi; I only found an annotation referencing it in the Sanguozhi pdf I linked. Nevertheless, the major battles described generally occurred, and people fought for who the book said the fought for, even if not for the exact same reasons. The portrayals are heavily influenced by the culture and time period in which it was written, but so were the other Chinese histories; it's just a filter you have to be aware of.

Kongming.net has this english translated biography of Lu Bu from Sanguozi: http://kongming.net/novel/sgz/lubu.php

Since I don't believe there is any english translated prints of Sanguozi, can someone here comment on the accuracy of the translation?