I visited Giostra at the beginning of an italian vacation that included many excellent dining experiences. We were very happy with our meal, the ambience and the service here (as the previous reviewer mentioned, all of the patrons at this small restaurant appeared to be tourists).
It should be noted that as part of the service charge, they give you a large antipasto platter. Also, the secondi are HUGE, so you might want to consider ordering a pasta, then sharing an entree.
In the end though, there were other places in the immediate vicinity that I preferred to Giostra. La Pentola dell'Oro, for instance, is just around the corner (on Via de Mezzo) and we preferred our meal there in an atmosphere that was a bit less touristy and at a nicer price point.