As above. Business cards are simple, practical and an effective means of communication. We are visual creatures, and the 'hi tech' alternatives - exchanging ecards/details using mobile phones - have never come close to replacing them.
Design is pretty subjective. Ideally, they should say something about you and your target market. A lawyer or an accountant will probably use plain cards because it fits the professional 'image', but a graphic designer or photographer might prefer something far more colourful and artistic, providing it isn't so complicated or 'busy' that it defeats the purpose.
I work in corporate security consulting and use plain, very high quality, cards - black print on white linen stock - with my name and contact details, that's all. It works for me, but might not be the best choice for you.
They're handy to scribble on too as others have said. I've lost count of the number of notes I've mislaid, after writing them on cigarette packets and beer mats!