Leone was a genious!
This is my favourite movie - and it's the start of the Once Upon a Time Trilogy which is totally different to the dollar trilogy. The dollar trilogy is fast paced action with one cynical climax after the other. The first one is a remake of Kurosawa's Yojimbo, as is another movie you've probably seen: Last Man Standing, with Bruce Willis. The 2nd has a bigger budget, a second star (Lee van Cleef, the perfect italo western actor) and is both more mainstream and more spectacular. Repeat and multiply for the 3rd. The trilogy ends with arguably the best showdown ever shot.
The first might seem a bit old nowadays, but imagine what an impact it must have had on a moviegoer used to seeing John Wayne and James Stewart in the clean Hollywood westerns.
The Once Upon a Time Trilogy on the other hand is slow, superbly written and acted and low on action. If you really need to compare it think The Godfather trilogy. The middle part has more action and is more chaotic, but that's the exception. All three movies have a message, while the dollar trilogy is just fun.
Once Upon a Time in the West's opening sequence is unique, for example. Here's a summary (not relevant for the main story): 3 gunslingers are waiting for the the train with their victim. Waiting, credits, waiting, a fly comes, credits, some waterdrops hit the ground, credits, waiting, credits, train arrives, victim is found, a oneliner, bangbang. The whole thing takes 18 minutes! And it's great. The whole cast is superb. Fonda is at his prime; Bronson, Robards and Cardinale have never been better. The few action scenes are full of surprise and suspense.
It is possible that you'll simply find Once Upon a Time in the West and Once Upon a Time in America boring and an hour too long. So make sure you're in the right mood.
If you like UuaTiA make sure to watch an unoffical Leone movie shortely thereafter:
My Name is Nobody. With Terrence Hill (without Bud Spencer) as Nobody who wants to become Somebody and Henry Fonda as aging gunman who wants to get out. Leone makes a parody on his most famous movie, including the perfect ending for his last western.
There are dozens of good italo westerns hidden in hundreds of mediocre ones. Let us know when you need more.