I did the IB between 1991 and 1993, at a state sixth form. As others have posted, we had to study three subjects at a higher level, and three subjects at a subsidiary level. One of the subjects had to be a second language, another a science, another a maths subject. There were a number of maths classes to account for varying maths ability - the one I did was no more difficult than GCSE, to be honest.
We also had to undertake a programme called "CAS" which is similar to a Duke of Edinburgh award. We had to carry out a number of hours activity in something creative, something active and a service to the community. Without fulfilling this element, the diploma will not be awarded.
Additionally, we had to undertake a non-examined course called "Theory of Knowledge," in which we spent many hours debating philosophy type questions and generally learning to think.
All students had to choose an topic to carry out an extended piece of writing (I think 4000 words) which I found to be execellent prep for university, as we had to choose the question ourselves and carry out our own research with very limited guidance.
The best bit, in my mind, was the international focus of the course. The syllabus is not taught from the perspective of United Kingdom, so the "english lanuguage" subject is actually called "world literature."
Although I failed the overall diploma (I failed the chemistry subject) my first choice university accepted anyway on the strength of my extended piece of writing. IB results are out a lot sooner than A Level results and they accepted me even before the A Level results came out.
Some people say that a good IB diploma is worth 4+ A Levels. I wouldn't like to compare, having limited knowledge of A Levels, but the course certainly was a challenge but also forced me to have a life outside the academics through the CAS programme. I'd encourage anyone down the IB route!