I applied for an intern position last spring but had been denied at the last stage. Square contacted me this fall and asked to interview again. Had 2 pairing interviews on campus that were very easy. Advanced for another onsite, this time in the new HQ. The morning consists of 3 more pairing interviews. These were more difficult questions, and the amount of pair programming vs. you just doing it alone varies completely by each interviewer. My first interviewer smelled of weed, didn't explain the problem very well, and I struggled with it while he sat on his own computer. I felt that he deliberately gave simple test data that concealed the complexity of the problem. Only after I started did I realize how I had messed up; short interviews and not much time to change that. One interviewer sat down and said let's make tic-tac-toe, and I had a blast hacking that with him in the 45 minutes time. The afternoon is odd. Some candidates are asked to do another interview, while the rest are given tech talks. It wasn't clear if they had advanced or what, and it was really awkward when they did that. Between the unprofessionalism of the 1st interviewer, the awkwardness of the afternoon, and the midnight redeye return flight they gave me, I'd say it was a slightly negative experience. The candidates are the best from the best universities (MIT, CMU, etc), so Square has the ability to be insanely selective. Even had I done well on the first one, I'm not sure if I would have received an offer.