It's sexy as hell... on a girl. I've had purple hair off and on for years. The darker is usually the way to go (rather than light purple) as you have more time with it as it fades before you have to redye it. Be warned though, its a lot of maintenance, but if you want to feel like a sexy rebel, go for it!
It could be awesomely cute. Or not. Guys don't decide to like or dislike a girl because of one thing like her hair color. If you like having purple hair, then do it. Make yourself happy. Happy people are attractive.