In my experience, it's excellent. I definitely found my IB classes a lot more intellectually challenging and invigorating than my AP classes. The curriculum is well-rounded and in-depth. Plus, the requirement of choosing only 4 HLs provides an automatic defense against excessive commitments.

I'm not sure how generalizable my experience is though. My AP classes were at a normal public high school while my IB classes were at an elite boarding school. I will say that the exams are definitely much better for the IB (they're much more detailed and require actual understanding rather than rote memorization).