Pointdexter, Hurri: Nearly everyone at my school were doing only the ACT yr 12. About 8 people in the school did both, becuase my school offers both IB and ACT yr 12, and ACT is appatently the only place in australia were you can do both. The workload was far greater than the other 'normal' people.
I only found out about the IB in year 10 when I was visiting the secondary colleges, and I thought it was a well balanced program, getting you involved in a heavy workload, lots of extra-curricular activities, and I looked at the conversion tables- most of the low IB scores converted to massive UAI equivalences. The other thing which really got me motivated to doing it was that because I would be doing both, I would end up with 2 UAI's (ENTER/TER etc) and so if I stuffed up one of the systems, the other one would be a backup. ( which didn't end up being the case)
Most of the things I had to learn for the ACT, I had to learn for the IB. Since the ACT doesn't have any final year exams (they are at the end of each term) and the IB does, it was all going to be just a big revision for my IB at the end of yr 12. However, there were a few subjects/subsubjects which didn't correspond to both, so I ended up doing 7-8 subjects in yr 11 and 12. Furthermore, the IB had a lot of external assessment, which was a pain, because it was all during mid year yr 12, when I was trying to focus on ACT assessment, UMAT practice, and application forms (I applied EVERYWHERE.) But the problem was most of the external assessment count towards about 30% of your subject grade. There were also huge assignments for the IB which took up a lot of precious time, such as the mandatory 4000 word extended essay (I did in HL Physics,) and the 'Theory of Knowledge' essay, which counted towards bonus points.
However, I am now redoing yr. 12 and NOT doing the IB. But there is a catch to it, because I can use my 2005 IB diploma as a backup when applying to uni's, and if I get a 99 this time round I won't be needing any of the scores from last year.