I went to the one tonight with O3 and it was great! Would highly recommend attending as a participant or as a spectator.
They give you a prompt and you work in a small group (5 or 6) to find problems, decide on a problem, iterate, and present a final solution in two hours or so (basically the whole design process but really fast). We had a moderator in our group who made sure we stayed on time and on topic.
Spots are still available for another one from another organization for tomorrow night (9/22):
https://www.eventbrite.com/e/phillychis-14th-annual-design-slam-tickets-118503377793