I always took advantage of the student host option whenever it was presented and I loved it for all the reasons that have already been mentioned (cost, getting to know students, finding out the "real story" about the program, etc) and also found it a lil stressful for all the reasons mentioned above (kinda have to stay in interview mode, little to no alone time).
At my school, students (regardless of if they are a host not though hosts tend to have a much greater base of information to draw from) are encouraged to share whatever especially positive or especially negative impressions we have of applicants. Those who are good guests are often helped by their hosts, and those who are rude/inappropriate can very much be hurt (ie: the applicant who helped himself to EVERYTHING in the fridge/cupboards with absolutely no compensation of any kind for all that he took - his host sure had a lot to share with the admissions faculty).
Some things to DO and NOT DO if you are staying with a host:
DO ask their perspective on the program (and other questions you may have about the program, city, etc)
DO be friendly and kind with your host and anyone else staying there (roommate, another interviewee)
DO NOT assume you can take any food/beverage you want
DO keep your stuff neat, tidy, and out of the way
DO be honest but also appropriate with your host (I don't think any host will believe you if you say you aren't nervous at all. When asked, I would admit that I was nervous but also excited. Didn't want to seem like overly nervous/neurotic/catastrophizing interviewee
)
DO clearly communicate your travel plans (when you plan to arrive, when you plan to depart, and maybe this was just me but all my hosts asked for flight numbers b/c you never know when a flight might be delayed. Also - make even small travel arrangements ahead of time. If you are getting a rental car, tell your host so they know. If you need transportation to campus from the host's place if it isn't within walking distance, I would typically ask about public transportation so your host doesn't feel obligated to transport you personally. I think they generally do the vast majority of the time, I just never wanted to really force the issue).
DO NOT be afraid to ask whomever is arranging it if you can have a host for 2 nights. I needed it a few times and never encountered any problems (I always phrased it so that they knew that I would prefer to have a host for two nights but if that wasn't possible, either night individually would be fine so at least one night would be covered).
If you have any questions/comments/concerns/etc about being hosted, feel free to PM if you don't want to post here
I'm more than happy to help!