Apache OpenOffice is easy to learn, and if you're already using another office software package, you'll take to OpenOffice straight away. Our world-wide native-language community means that OpenOffice is probably available and supported in your own language. And if you already have files from another office package - OpenOffice will probably read them with no difficulty.
That is the only problem I have found with open office, it is NOT completely compatible with microsoft office so sending documents from one source to the other is not always possible.