I've come here three times for events -- twice for Postcrossing meetups and once for Comic Invasion…read more-- and I can say it's a great spot for events. The building is old and beautiful and well maintained, with a large open floorplan that's perfect for hosting events.
The museum itself is also pretty cool, with dedicated sections about the history of the German post office, things like phones and computers, robots, etc. etc.
They have a rule about no backpacks which is kind of weird to me? But they do have lockers and bathrooms. The gift shop is also really cool, with toys, books, postcards, and such. All in all, worth a visit.