The inside is very conducive to collaborative work. The offices are organized around the periphery of the building, and are functional, but not overly so. The idea is that people will congregate in the large interior open spaces that are covered with whiteboards and comfy chairs.
This is what the interior looks like: (apologies for the picture quality)
The second floor open lounge
Comfy chairs in front of a whiteboard
even more chairs and more whiteboards
Let us not forget the very fancy coffee machine (which makes nice espresso)
and of course you need your Simons Institute mug to drink it in.
This is the main auditorium for workshops, on the first floor.
The next plan is to pave the outer walls of the building at ground level with chalkboards, so people can think and chat outside as well as inside. Ah, Berkeley weather. I'm told the boards are already there, and just need to be installed, so I hope to have pictures soon.
There are 93 visitors between the two programs (53/39 by my rough count) which includes 22 Fellows. Offices are either two or three-person, and there are some large cubicle spaces with 7+ inhabitants. The visitors are all on the second and third floors of the building, which both have large open areas (the pictures above are from the second floor).