I have stayed here on business -- again could never afford it on my own. Although the rooms are a bit small, the are well equipped and the hotel breakfast is far and above anything else I have enjoyed in the city.
I normally prefer to stay near the Fontana di Trevi or Piazza Navona, but for pure luxury, you can't beat the Sofitel.
The resaturant and bar are extremely good places to just hang out and people watch.