All the time! We like the City Circle and the kids love it too, albeit, it is very slow to get from one point to another, and the City Circle is always full!!
I like the fact that there is a recorded commentary pointing out all the landmarks for the tourists. I also think it is nice of the City of Melbourne to provide a tram ride for tourists that is not heavy on their pocket (i.e. it is free). The historic aspect of the trams also adds to the charm of the city visit.
I have been on it a few times. Melbourne now has a lot of the regular trams going around & through the city that are faster & also free, in the city sections. I live in Sydney, but visit family in Melbourne, 4/5 times a year.