In the three years that I have been studying and speaking Italian, this is the single best dictionary that I have come across. I have several others (Oxford, Langenscheidt's, etc.) which are all fine, but don't come close to the Websters. The Websters includes practical examples, idioms, verb conjugation and several other useful tools that really make it top notch.
I spent the better part of a year at a language school in Italy, and the Websters was invaluable. When I was preparing to return to the U.S., my instructors asked if I would leave them my copy!
I will concede that the Websters is too big for carrying on your travels, but if you are studying or corresponding with Italians, you simply cannot find a better dictionary.