I am currently at the Rome Central Park Hotel for a long-term stay and would recommend it strongly. I travel frequently to Italy and France and consider the Grand Flora to be one of the best hotels anywhere. But although I've stayed there four times in 2009, it's expensive and it's very difficult to use Marriott points as many members have pointed out.
This is my first trip to the Rome Central Park. No, it's not central, but it is in a park (Pineta Sacchetti). If you get one of most of the rooms that have a park (and Vatican) view, you will have a lovely balcony, very business friendly and large room, air conditioning (that works!) and a much less expensive price tag.
I too was concerned about distance from Il Centro. But I haven't even used the complimentary bus to Ottaviano which runs every hour from the hotel and every half hour back. If you walk right around the corner (less than 5 minutes) you're at the local Gemelli train stop. Take the train for the same price as a bus or metro (1 euro, or better yet a multiday or weeklong ticket) in the direction of Ostiense but transfer at Valle Aurelia and you're on a main metro line. (It's sometimes taken me longer to walk to the Barberini stop from the Grand Flora than taking the train tothe center from here.) Another added convenience is that directly across from Gemelli is a Supermercato. So if you need anything you forgot (or wine or cheese or fruit, it's right there).
So I would add to what others have said. When I can stay at the Grand Flora I do, but this is a great alternative at much cheaper rates.
The staff are very friendly and all speak English and accommodated me immediately for an early arrival and I've had good meals at the restaurant.
My only gripe is the cost of wireless access, which I usually get free at the Flora.