I found ES trains in Italy to be crowded when they were on unique, long-distance runs but not when there were many other options. The downside to having reserved a particular train, which is necessary on the ES and EC runs, is that if it is seriously delayed, you have to exchange your ticket at a station to get a different route. And indeed Trenitalia does have lower cost options for advance purchase and it does not have a fee. Be familiar with the schedules. I tried to avoid Eurocity trains (old and uncomfortable compartments are common - long waits in stations is usual) and favored Eurostars (optimal comfort and speed) and Regionales (low cost and very flexible).