Ravenclaws are smart, love to learn, and value knowledge above all. Hufflepuffs are loyal, hardworking, honest, and accepting. Gryffindors are brave -- sometimes reckless -- and value courage. And now for the House everyone seems to hate (though admittedly, there are good reasons for its reputation):
Ambitious, cunning, resourceful, with "a certain disregard for rules" -- these qualities describe members of Slytherin House. And as Hagrid (incorrectly) said in Harry Potter and the Sorcerer's Stone, "there's not a witch or wizard went bad that wasn't in Slytherin." Founded by Salazar Slytherin, it has included Tom Riddle/Voldemort (Dark Lord), Bellatrix Lestrange (Death Eater), and Lucius Malfoy (rich, arrogant ass).
However, it has also included Phineas Nigellus (Hogwarts Headmaster, if a rather unpleasant person), Horace Slughorn (the quintessential college professor), Draco Malfoy (sympathetic if foolish and master of the Elder Wand), Narcissa Malfoy (the only reason Harry didn't die in the forest), Severus Snape (Death Eater turned good guy), and Regulus Black -- another Death Eater turned traitor, and a very unusual Slytherin indeed. It's a unique pureblood who's kind to house-elves, turns against Voldemort, and sacrifices himself at the age of seventeen to destroy a Horcrux, rather than letting Kreacher die. I mean, come on. Regulus is more of a hero than Sirius ever was.
Anyway, being in Gryffindor doesn't automatically make you a good guy, as Peter Pettigrew demonstrated. How the hell he got into Gryffindor in the first place beats me. Even Dumbledore, a Gryffindor, was tempted by the Dark Arts before his sister died...as a direct effect of his plans to take over the world. Percy Weasley is a Gryffindor, when his ambitions would seem to fit better with Slytherin.
Godric Gryffindor himself also stole the Sword of Gryffindor from the goblins. He was BFFs with Salazar Slytherin before their disagreement about letting Muggle-borns into Hogwarts. It's safe to assume that while Godric didn't agree with discrimination against humans, he had no qualms about treating other races badly. And really, can you blame the original Slytherin for not trusting Muggles and Muggle-borns? This was in a time when Muggles were actively persecuting and burning witches and wizards. The students they accepted into the school could have gone back to their families and been killed by them. They could have told their parents where Hogwarts was and potentially led angry Muggle mobs to the gate. If he wasn't at least a little wary, he'd be foolish.
All in all, the seventh HP book goes out of its way to blur the lines. It seems that when Slytherins are bad, they're really bad. But when they're good, they're very good. Look at Regulus and Snape. After all, as Dumbledore says in Chamber of Secrets, "It is our choices that make us who we are, not our abilities."
So if I had the choice, I would be in Slytherin just to spite everyone who hates it. ;) And being ambitious is not a bad thing. Add that to the cool, kind of goth common room and my favorite color green, and yeah, I would choose Slytherin. This is all hypothetical, of course, since I'm a Muggle. And would probably be eaten by the Basilisk, so...yeah. That part of Slytherin definitely sucks.