I completed the full-diploma programme of the International Baccalaureate program just last year.
Although it is supposed to be a standardized programme, your experience in it will be based on a large number of factors. What subjects are offered at your school? What is the attitude of people in that school to that programme? Is it considered elitist and exclusive? Is it a strictly IB school? How experienced are the teachers at teaching the IB curriculum?
Now it's very difficult to know the answers to those questions before you experience it. For people who didn't enjoy the program, it may be because the students were hyper-competitive, the teachers were inexperienced, and they had very few or no choices. People who enjoyed it may have had great, experienced teachers, supportive and friendly classmates, etc. One advantage is that you are more likely to be placed in the company of people who are like-minded (theoretically, at least, if you're doing the IB program you are quite good academically, and will have less to worry about with respect to group projects, bookworm persecution (not that all IB students are bookworms), etc.)
In my opinion, your skills at writing essays will vastly improve due to the IB program, because you will do a lot of essay writing.
I don't have a tonne of time to explain everything in detail now, but if anyone has any questions about my experience and opinion on this program, just ask and I'll do my best to answer it, although I will reiterate that everyone's experience will be different based on personal disposition, school, peers, etc.