ok, the guy to ask about IB is me, because I graduated with that program. Now, the following info may be outdated a little as they keep changing IB, but here it goes anyway.
First of all, it is very useful. But that is, if the school actually offers most of the courses needed. To get the diploma, you need 3 high level and 3 standard level courses as well as a certain amount of CAS (community service) and TOK (Theory of Knowledge, ethics, philosophy etc).
Now why is it useful? Because you can get universities to exempt you from certain courses if you achieve a high grade in IB from the same or similar courses. As an example, I got exemted from my first year physics course, 72 units of Social Sciences (thats a whole years worth!) and would have got exempted from Java programming if I had bothered to turn in my course description to the registrar

If you choose the right courses, you can get exempted from as much as a whole years worth of university courses, (some of my friends did). For example, lets say you want engineering. Take high level chem, physics and math and thats a year gone!
The course choices are as follows (no you can't take all sciences and no literature!) You need 2 languages (A1, and A2), a math course (there are 3 levels actually, 2 std and 1 higher), a science course (chem, phys, bio) and one elective (any one extra from above or a totally different course).
As in which universities accept it. Nearly all of them! Europe, UK, USA, Canada, Australia... They may not SAY that they accept it, but if you provide it anyways, you are almost guarenteed to get credit somewhere.
Also the final IB grade is I think out of 45. You get 1-7 for each course and then add them up. Most universities, especially in the UK have a min IB grade you need to get to be able to enroll (usually around 32).
So yeah, I say it is a very useful course, if you have any questions, reply or PM me. btw, for me it was much easier than GCSE, concentrate on the high level course (you get 2h more of them) and you are almost guarenteed to get a high grade.
edit: btw, our school was one of the few schools in the world which REQUIRED you to take IB, so I didn't have much a choice. But it is becoming the standard education program worldwide.
edit 2: oh, didn't mention, I'm Turkish, took IB in Turkey in an international school and now I'm studying Computer Engineering in Canada.
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