My 2e son (who has dyslexia and ADHD) was in an IB program in a very small school for one semester. It was not a good fit for a number of reasons. The overarching issue was that for a STEM focused kid, this particular school had weak STEM related coursework and opportunities (for example, they did not offer HL math and the only IB science they offered was biology). On the flip side, the students were forced to take HL English and history, which also didn't sit well with my son.
Like Platypus101 said, the workload was intense, and made more intense for my son because reading and writing took him much longer to do. The executive skills required were over the top as well--not only did students have to juggle eight classes, they had to prepare an extended essay and engage in a certain number of hours of extracurricular activities.
So on top of working all the time on things that he wasn't interested in, he had no time to pursue his interests after school. So we withdrew him from the school and he ended up dual enrolling at the local community college instead. Looking back on it, it was absolutely the right decision, but at the time it was pretty frightening--as withdrawing from high school in the middle of one's junior year is generally not the done thing.
One other thing you should know: The IB people don't seem to get LD accommodations (or at least they didn't--maybe they have been enlightened recently). Apparently they will only give a 125% extended time accommodation on their exams. They said it was because any more time than that would make the student "tired" or some such nonsense. So that is something to consider as well.
