This is an amazing fountain, which like many of the fountains in Rome, is still powered by the original ancient aqueducts running into the city. (The aqueducts were destroyed when the Goths sacked the city, but one of the popes had them rebuilt during the Renaissance.) Every time I visit, I am amazed at how such a large fountain fits into such a tiny square. If you press your back against one of the shop windows opposite the fountain, you still can't fit the entire fountain into the viewfinder of your camera.
The fountain is very romantic at night when it is lit by the surrounding floodlights, however my husband was so bothered by the guys selling roses to anyone who remotely looked like a couple that we only stayed for about 5 minutes. I might consider buying a rose just to get them to leave you alone, or pack a silk one in your suitcase to carry with you. They are also all over the Spanish steps, Piazza Navona and Campo dei Fiori.
Legend says that if you toss a coin over your shoulder into the Trevi Fountain it will assure your return to Rome one day.