I did a set of interviews at Box very recently.
My first impression of the place is that it's a nasty commute to get to, if one doesn't live close by on the Peninsula. Traffic to get to & from the office on El Camino Real seems very likely to be a snarl during the day, and if you're doing Caltrain, be certain to catch the right train, or else you might not make the shuttle you were hoping for.
I did see the giant slide in the lobby from the 2nd floor. According to the front desk receptionists, it's rarely used. Before going into your interview, you can pick up a few cold drinks (sodas, water, etc.) from the cooler that's right in the lobby.
The work environment is of the large room with worktables variety (i.e. no cubicles, minimal personal space and nowhere to hide where one can just concentrate on what's in front of you). Various meeting rooms and server closets (presumably) break up the workspace areas a bit.
Interviews are conducted in those meeting rooms. They have the glass walled "fishbowl" kind of meeting rooms (where everyone walking or rolling by on scooters can peek in and see you doing your whiteboard dance) and others that are four solid white walls (where you write your whiteboard answers on the walls… the scrawling from the markers doesn't clean off the walls super well but they can be erased with some effort).
They had a few people from my potential team talking with me and then a couple others from a non-related team. One of the interviews was a team, where one of the interviewers had a silent partner sent along to observe (and learn) how Box ritually slaughters their candidates.
I got to step into the recruiters area and they have a white board full of names where offers have been extended, indicating they are hiring aggressively (which also can be seen as a potential danger sign… binging on a large amount of new hires can lead to purging when today's dot com 2.0 boom times turn bad).
Don't assume getting in here for an on-site interview means a job at Box is a shoo-in though: even though Box is apparently in dire need of help with my expertise (and the team I interviewed with is supposedly short staffed for the amount of work they have on the books to do), I clearly fouled up at least one of my interview answers and that's most certainly what got me vetoed from getting an offer. Study up on specifics closer to whatever type of developer role your interviewing for, but also be ready for general computer science topics (ala traditional Apple or Google interviews). Good luck!