The Prince of Swords
One of the most common interpretation of this card is that it represents a dangerous or treacherous man - which, IMHO, is a very superficial way of looking at a Court card.
Certainly this Prince can be sly, dishonest and untrustworthy - but only when badly dignified by the cards around him. The card can also sometimes come up to mark a person who is angry, or vengeful.
But the pure Prince of Swords type is a highly intellectual and usually well-educated person, with a rapid fire mind and a great capacity for abstract thinking. He produces ideas with astonishing speed, but often moves on too quickly to follow through or elaborate on them. He can be challenging, entertaining, stimulating - and completely exhausting!
The card represents a private person, who defends his inner space quite determinedly. This is some-one who is hard to get to know - in fact, you'll probably not succeed entirely no matter how long you know him. He is a thinker, and chooses those he shares his thoughts with carefully. He's usually also very independent, and often appears unemotional and cold.
Sometimes the Prince of Swords will come up to represent somebody who is embarking on a serious course of occult study - with the Knight indicating the dedicated initiate.
The bad reputation comes from one peculiarity of this card and the Knight of Swords, I think. They both tend to appear when a man is angry, violent or vicious. However this is a function of the Suit - Swords deal with conflict and pain quite extensively. So don't imagine that every Prince of Swords you see is bad - most of them aren't.