I finished by IB diploma 5 years ago. In retrospect, it was a pretty valuable experience - though not because it prepares you for university. It only opens doors for you. Just think about this: 10 years down the road, would you rather have worked hard for 2 years to get into an elite school or not?
Also, to everyone who thinks IB is a lot of work, just try university. IB is nothing compared to the workload you will get in 2nd year and beyond. Don't even get me started on masters programs.
In my view, IB is a relatively short-time investment for long-term gain. To be fair, your social life will suffer compared to the rest of your non-IB friends, and you will likely spend more than all-nighters getting ready for finals. But, If you are willing to work hard now in order to reap gains later then go for it.
On a side note: the Theory of Knowledge class is invaluable - you will be using that shit forever. I still benefit from that class.