My school transitioned from AP to IB a couple of years ago, and now I’m a full IB Diploma candidate. Overall, I like the program.
Regarding IB Math: HL Math is, without a doubt, my most demanding class. There’s a lot of material to cover, the notation and problems can seem weird, and the topics are quite advanced. For my school at least, HL math is a “double HL.” SL math is notably easier but still rigorous. In my opinion, this is the class most people should take. If you are exceptionally good at math, take Furthers. If you’re good at math/might major in math/want a very advanced math background, take HL. If you like math but don’t want to spend tons of time on it, then take SL. A couple of kids in my school are taking Studies, and even they wish they were taking SL at times.
Regarding Diploma vs. No Diploma: Get the diploma. I don’t understand why someone would accumulate a bunch of certificates when he/she could be getting the full diploma with relatively limited more work.
Many of the above posters pointed out the positives (and negatives) of the IB. I would add that the Extended Essay is a great opportunity to not only prepare yourself for college, but also to showcase an academic interest. If executed properly, the EE can be a published piece of work, go way beyond the classroom level, allow you to connect with professors, etc. To a smaller extent, the Internal Assessments can be used effectively as well. For example, for mathematics, I investigated the math behind the golf swing.