I lived in Rome in 2000 (the Jubilee year) and can testify to the fact that the city was the most crowded place I'd ever seen in my life that weekend. I took a train away to Elba after I realized it wasn't going to be easy going.

However, those who stayed in town and went to the Vatican for Easter said it was a wonderful experience, albeit immensely crowded. It helps, I think, if you begin writing now to get primo seats for Easter Mass (as I'm not Catholic, I may have mis-named the big Easter ceremony). As many of the Easter visitors to Rome are pilgrims, most non-pilgrim places (like good restaurants in Trastevere, and Testaccio, some of the more off-beat attractions, like maybe the catacombs or some of the smaller museums) won't be excessively crowded.

Maybe someone who experienced Rome more as a tourist or in a non-Jubilee year can provide more information.