I have had the pleasure a few times thanks to being an escort and polyamourous.
The fun really depends on the people involved. I had a 4sum with 2 other providers and a client I didn't know well and it was absolute insanity in the best possible way.
But being with my partner and another couple lead to a lot of jealousy and people feeling left out.
I think the client session was so fun because there was no emotional investment. We were all just vibing and egging each other on. Where as the 2 couples were just not a good match in terms of how we all 'did poly' and set boundaries.
So, when it's bad, it can be really bad, butt when it's good? It amazing!