Lü Bu was a real person, yes. Chen Shou's Sangouzhi (三國志, Records of the Three Kingdoms, written in the 3rd century) mentions him, and that's pretty much the most cited historical document for the time period/location.

While the site itself is mostly dedicated to Luo Guanzhong's Sanguo yanyi (三國演義, Romance of the Three Kingdoms) and the games by Koei Corp. (Romance of the Three Kingdoms, Dynasty Warriors, etc), it does provide English translations of the Sanguozhi, and here is Lü Bu's:

http://kongming.net/novel/sgz/lubu.php

From my reading of it, it seems that yes, Lü Bu was very much a skilled and powerful warrior. Luo Guanzhong may have been a bit biased towards painting Liu Bei in a sympathetic light (Liu's pity towards Lu Bu, wanting Liu to appear gentle), and in the novel Lü seems to be a bit more... brutish and stubborn, which the video games definitely capitalize on.