I heard a lot of things about him.. But.. My small insight on the character.
Many treat him as just-another-member, yet I think of him as "the only optional character in any RPG ever made that you can miss, and by missing him, you miss one of the most important characters ever". Some just bring him down to a guy who was experimented on, just-another-hidden-character-dont-need-him-to-finish-the-game, period. Well, it's as bringing down Sephiroth to a crazy-mad-guy-who-likes-to-kill.
*SPOILERS AHEAD*
So.. Vincent's storyline as him being a Turk is quite known. Yet the experiments that Hojo performed on him and their results were never clearly explained. That he was implemented Jenova Cells - I have no doubt. The reason he always is so clothed is because of countless scars left by Hojo when experimenting on him - I guess the golden arm and leg are just a addition he put in, after checking "let's see when more I can do with him". Though I doubt he expected such results from Jenova Cells living inside Vincent's body. He can mutate - and these are the main reason for that.
Going back to "current times" of the main storyline, many see Vincent as a vampire. Well, I think he is eternal, and that's the saddest part on his character, but he is far away from a classical vampire. Whatever Hojo planned, it turned out this way, doubtfully what Hojo wanted. As a random side-effect. Vincent knows much more then he should know, and I'm quite positive he posesses the wisdom of the ancients. Think about Sephiroth.. he was given birth to by Lucretia, with countless Jenova cells implemented in him in prenatal stage. So he became a sort of a hybrid between a human and Jenova. Vincent was given birth to, lived, and then was forced to become a clone, so he is demi-human. With being eternal, and the wisdom he posesses, he decided to leave the world, and bear eternal rest, yet one thing moved him - it wasn't Sephiroth exactly, I think it was the need to understand himself who he really is. And Sephiroth, as a sort of a "brother" might've known the answer.
Trailers from Advent Children suggest even far more on Vincent's knowledge and the mystery surrounding him. I guess DoC will shed light on him too.
*SPOILERS END*
Just something I came up with.. =)