I have done IOP on two occasions. I didn't find the first time to be that helpful, but that group was almost all people a fair amount older than me and I was having trouble relating to their struggles, plus I was still not in a place where I necessarily really wanted to or even knew how to change. Then I did IOP again a few years later after a stay at a crisis center. My therapist got me into an IOP group with people more my own age (it was a specific young adult group) and I not only was better able to relate with the others in the group but I also had been making some progress before, so I already kinda was able to access that growth mindset and wanted to get myself back on the right track.
So I'll say the IOP experience can be really variable, and that can depend on factors inside and outside of yourself, like the other people or the facilitators, and your willingness to learn and grow. You can really get a lot out of it if the conditions are right. But of course there's no guarantees.