I haven't stayed at this property either, but I would certainly consider staying there on my next trip. Other hotels worth considering include the De La Ville Inter-Continental (in spite of the extremely dirty facade I saw last year), the Ambasciatori Palace (located on the same street as the Marriott and is a member of the French Concorde Hotel chain), and the D'Inghilterra. These three hotels are probably somewhat pricier than the Marriott. The Hilton is just too far from the city center and the Crowne Plaza has an impressive lobby but plain rooms.
When in Rome, beware of pickpockets (especially gypsies and children finding ways to distract you). I ran into some near the Vatican museum but did not lose my wallet. For this reason, I have avoided taking buses (especially the number 64). I have used the metro/subway system a number of times and found that many people (Italians included) do not know that they can purchase more than one ticket at a time from the machine simply by using the "+" button. As a result, I spent a lot of time waiting for my turn at the ticket machine at the Spanish Steps station. While the metro system is efficient, it is rather inconvenient because only the Spanish Steps and the Colosseum are located right next to a metro station. To get to the other main tourist attractions, you can expect walk quite a bit.
Finally, don't miss the Borghese Gallery, which reopened about a year ago. It's located within the Borghese gardens very near the Marriott. To visit the precious art collection, you will need reservations.