We stayed at the Hassler for a week between christmas and new year's 2012. For a 5-star hotel in the heart of the city, we cannot recommend this hotel enough.
The hotel is in a perfect location at the top of the Spanish Steps. The views are fantastic and many historical sites, lots of shopping and other places of interest are only a short walk away.
We found the staff at the hotel to be excellent. They are very attentive, well-trained and exceedingly capable. They seek to ensure every guest is happy and they succeed.
We stayed in a street-facing grand deluxe room which was very comfortable and gave us a nice view of the city. The rooms housing the classic or more elaborate suites likely had the better views, but we were happy nonetheless. Rooms have great robes and slippers which are welcome after exploring the streets and sites for hours at a time.
The staff working the breakfast buffet in the Salon Medici were outstanding. By our second day, they knew our preferences, etc. My wife and I didn't have to ask for our coffee drinks to have soy instead of cows milk after our first breakfast. Nice!
Overall, we had a great stay and would most definitely recommend this hotel to anyone that wants great service in a perfect location, particularly anyone who is visiting Rome for the first time.