I have had my own poopy diaper changed once and have changed poopy pants/diapers a few times. I can share my experiences.
The first time ever was hanging out with an ABDL who lived semi close, we'd never met before, but we had a "littles" playdate. It was kind of underwhelming and hard to get into the mindset because we barely knew each other. We both messed and it was a bit lackluster on both ends, for me because my mess was not a fun consistency and wasn't that much, despite holding for a couple days. And his wasn't really that pleasant smelling, compared to my own and what I typically fantasize about smelling.
I also had a girlfriend who messed in a diaper twice for me, but both times she didn't enjoy it too much and didn't want to do anything fun like sit in it or have me touch her bottom. All in all more work than it was worth.
All that being said, I'm still holding out to more experiences, hopefully better ones. It's still one of my most exciting fantasies.