I spent 3 days in Rome in late 2003, and I decided to skip the zoo as I had heard dreadful things about it and there were simply too many other attractions that I deemed more important. From what I have read and heard from fellow ZooChatters it seems to have improved in the 6 years since I was in that beautiful city.
By coincidence my own previous visit was in 2003, and most of the better exhibits nowadays (chimps, bears, lions) were already there. The Reptile House and Giraffe Paddocks have been refurbished since then and the pleasant BiodiversItalia display of native species added, but there have been no other grand changes that I noticed (maybe the Children's Farm? That looked quite new). Just a general tidy-up and a lick of paint!