Not sure you need to spend a lot of time in the piazzale itself, because there's not much there besides the parking lot, but the view is totally worth it. You can get a panoramic view of pretty much the whole city, the river, the domes, etc. When we were there it was raining (pouring) on and off, but when it was raining the city was beautiful and when it was sunny the whole vista was spectacular. Worth the climb up. The gelateria up top is pretty tasty, too, and there's the chapel at the top of the hill that's worth seeing.