I did the IB (crikey) 17 years ago at a state sixth form college in the UK. My cohort was the first to study the IB at this particular college and as there were only 5 of us on the course we probably had an education that couldn't be bought elsewhere! However, our teachers were also new to the IB so I don't think any of us performed particularly well.
I found it an extraordinary experience. I enjoyed studying a broad range of subjects and particularly enjoyed the fact that it wasn't UK centric, for example, we studies World Literature not English Literature.
CAS (Creativity, Artistic and Service) was a great diversion from the academic side and I did a load of things that I probably wouldn't have prioritised over study at that time. But as it was an essential part of the course, we made that a priority. We also had to write an extended essay in a subject of our choice on a topic picked by ourselves, which I personally found to be good preparation for university and dissertation writing.
We also had to study something called Theory of Knowledge which was supposed to be about how to think, learn, debate, that sort of thing. I enjoyed it but to be honest never really got my head around what it was supposed to be about. Maybe it was too much of a difference to GCSE, I don't know.
If my DS is an all-rounder, and someone who is great at everything, I'd encourage him to do the IB too. Personally, as someone who struggles with maths and sciences, it probably wasn't the best choice for me, but I was lucky in that we had almost one to one tuition.
I had offers from all the universities I applied to, and they all gave me low offers when compared with A Level students. I suspect that was because back then no-one really knew much about the IB!