I like a full day interview because it allows me to get a good look at the office, the environment of people around it, and meet 4-5 prospective coworkers and get a fuller assessment.

However, the day is truly grueling. I interviewed @ Google Cambridge a few years ago (spoiler: did not get the job) and was exhausted by the end of the day I could barely think. My throat was parched from basically performing some sort of stream of consciousness dialogue so that my interviewers were able to assess my problem solving technique and going through corner cases. It's a marathon for sure..