YIP (Yelp Intern Program) is awesome and their dedicated team for handling interns is the best I've come across. They host weekly events that allow you to really get to know the other interns and explore San Francisco which is honestly a major plus.
That being said, the work depends on what team you end up on and what projects are available. I personally did not get much mentorship and worked independently on an intern project for most of the term which does not give me a good feel of what it is like to be a full-time at Yelp. However, some of the other interns had good experiences and caring mentors, so that is not to say you will not find any meaningful work.
Culture-wise, Yelp is great and all the people I met and interacted with were friendly and made me feel welcome. The people at Yelp are what makes working at Yelp awesome in my opinion.
Overall, I had a great time as an intern and would definitely recommend it for a coop just because of how well run the intern program is!