This is the only Leone movie I've never seen - anyone recommend it?
Welcome Aboard!Colossus of Rhodes
is a "Sword and Sandals" movie -- a genre which I have no interest in.
I don't think it was a great movie, but then again, I have not seen any other movies from that genre; I only saw COR for the sake of seeing every Leone movie. If you like that genre, maybe you will like COR as well.
You're not missing anything if you do not watch it; I'd only recommend watching it if you are a big Leone fan and want to see it just for the sake of seeing all of Leone's movies. If you do want to watch it, I'd say you should first rent it rather than buying it, cuz there is a good chance you will only wanna see it once. And if you do rent or buy the dvd, check out the commentary by Christopher Frayling; it is always a joy to listen to his commentaries, cuz he knows more about Leone than any person who ever lived