A few years ago, I stayed in Casa Kloster for about a month, while getting settled into the city for a job teaching in a language school. The price at the time was extremely affordable (about $6 US per night), and the downtown location put me within a close shot of the school, as well as my favorite stomping grounds. The rooms and bathrooms (you share) were always quite clean, and a bedside jar of drinking water was refreshed daily.
Probably what was most enjoyable about my stay were my fellow travellers. The hotel is regularly frequented by backpacking travellers from Canada, Australia, all parts of Europe, and other locations. Many of my most enjoyable times were spent sitting around sharing travel and life stories with this group of people, and one fellow I remained in contact with for a few years afterward.
If a person is looking for deluxe resort accomodations, this isn't the hotel (and probably not the city - stick to Cancun), but for those wanting a more than affordable stay and a great time, I would recommend Casa Kloster most highly.