Yeah, you really don't need to like Westerns to like it...but it is a Western. In the Gene Hackman thread a few weeks ago, you got the feeling 99% of the board has seen Hoosiers but I'm sure 99% of the board would love Unforgiven if they had somehow not seen it.
Stuff like High Noon, The Magnificent Seven, or The Good/Bad/Ugly are all objectively great movies too but I'd never tell someone to watch them who "doesn't like westerns" because I know they wouldn't like them.
Once Upon a Time in the West I'd recommend to someone who just likes amazing achievement in the arts. It's just a beautiful looking and sounding thing. It has parts where that beautiful vision is tough and violent, but other times where the beauty is romantic and inspiring.