Not clean, awful location, expensive
Maybe my expectation were too high based on what you would get in the US, but for $600/night we got a small room that was beautifully decorated, but not very clean or comfortable. I've seen some users mention the terrific beds and bedding, but we had sagging beds and sheets like you get at K-Mart. They also tried to nickel-and-dime us for everything like high-priced internet access and $60 for a breakfast in bed. There was no iron or kitchenette or even an electric kettle.
One thing we noticed in Italy is that they will definitely judge you on appearances. My wife and I were definitely younger than most other guests and were dressed down and a bit sweaty when we checked in (just finished a 12-hr trip). I think they took this as license to treat us poorly (but charge full price).
The staff were curt in our limited dealings. I wanted to give the desk clerk a piece of my mind, but he was on the phone the entire time he was taking my money and didn't even care to ask if we had a pleasant stay.
The Via Veneto area has lots of top hotels, but not much else. Villa Borghese park is loud and more of theme park than a city park (terrible for picnics). If we ever go to Rome again, we would stay near Piazza Navona or Campo Di Fiori and we will never stay at a Westin (or Starwood) again.