We have them in Portugal too, actually. I think most european countries have some kind of national ID card. It has become a part of our way of life, and it doesn't bother us at all.
Some aspects of ID card usage in Portugal are:
You must get one once you're 12 years old.
You should carry it at all times when you're away from home. Foreign citizens should carry their passports.
It includes a photo, index fingerprint and official signature.
You must show it when requested by properly identified officers of the law. If you don't, you could be taken to a police station for identification. Nevertheless, most officers will trust a driver's licence if, for some reason, you are not carrying your ID card.
It is normally used for checking your age at cinemas and clubs. It is also used to check your signature on checks and credit card notes.
Most likely, it won't be accepted if it has traces of tampering or if it's damaged - that's why most people "seal" them in plastic once they get them, to avoid damage.
It is renewed every 5 years, demanding a new photo. It's the only occasion in which people are allowed to change their official signature.
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Some aspects of ID card usage in Portugal are: