I did Mid-IB at OSC cause I didn't trust my teachers and my 5 weeks there were well spent. You get examiners as teachers who can try to answer your questions (I had some they were unable to answer :-/). But generally they were good, especially the math and economics teachers I had there.
I imagine the pre-exam course is just a condensed version of the Mid-IB course so while I can't comment on whether they are worth it, a revision course is a good idea. There are cheaper alternatives as well in Austria or with Lanterna: Sweden, London and the US. At Lanterna you get ex-IB alumni who got 40+ points and they tell you how they did it and pass on as much as they can. You also go through the whole syllabus and practice many past papers, since I teach psychology for those I can recommend them highly as the students have lots of fun and appreciate being taught by someone closer to their age. It creates a unique atmosphere.
One last thing: Don't overrely on a revision course, it's no substitute for normal teaching. Revision courses are for clearing up minor things not fully learning large topics.