Mediocre at best though I suppose you come here for the "ambiance" anyway. We came here for "Trevi Time" but found out that you have to actually sit at the bar counter to get the HH prices. We opted to then pass on HH since we wanted to sit out by the fountain. The fountain is loud as others have said but it actually is cool having it as part of your view as well as being able to see the "sky" above change from day to night.
Food and drinks were just "meh." I had the frozen bellini and my sister had either the same drink or the Capri Coral Crush. One thing's for sure about both of our drinks: the drinks themselves while not being strong sure had a strong alcohol taste which we both found unpleasant. Thank goodness I had ordered gelato with mine or else I would not have been able to drink much of the bellini at all!
To eat we ordered the lasagna pizza since it sounded interesting and seemed to get good reviews. I personally wasn't too crazy about it. I was expecting more of a deep dish pizza taste to it and I've had really good deep dish pizza so I was disappointed to find out it was more lasagna than pizza, and cold lasagna at that. And I can actually say that I find Sbarro lasagna tastier than Trevi's lasagna pizza!
It's definitely fun and kitschy to sit outside by the fountain - just don't expect to be blown away by the food or drinks.