I stayed there for a week mid-June 2004. The hotel was quite nice - it exceeded my expectation considering what I paid and that it was branded Mercure (as opposed to Grand Mercure or Novotel). The room was new, clean, and not too small (the hotel was recently refurbished). The staff there were friendly especially the ones at the bar and breakfast room. The buffet breakfast was also very nice (again considering what I paid).
The location of the hotel is probably not the best if you are looking to see the sights. (It was ok for me as I was there for a conference at the nearby university). The subway station is about a 15 minutes walk away, from which it is a 15 minutes ride to get to the Termini, and 25 minutes to get to the Trevi Fountain area. The hotel began offering a free shuttle service when I stayed. It takes you to the Colosseum as well as the Termini, which does compensate for its location, but DO NOT rely on it. It does not run by its timetable -- in the sense that the driver does not wait and leaves before it is scheduled to!! On a few occassions, I got to the pickup point 5-10 minutes prior to the scheduled departure time and later found that the shuttle had left early and I had to get a cab instead! I guess you can get there 20 minutes prior to its timetable departure time to make sure that you do get it. I made a compliant to the hotel manager, hopefully they have done something about it by now.
I was at Paris just before Rome. We paid about the same amount (150 euro/night) for the Paris hotel, but this Mercure is just so much better in every aspect.
With the exception of the shuttle serivce, I was very happy with the hotel.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.