The plot for Showdown is so ordinary that you might just find yourself asking why you picked this up in the video rental place. But there's enough star power on the first through third tiers that this 1973 oater can't be a complete waste of time.
I remember seeing the movie on NBC some thirty years or more ago. I'd never heard of it, it had interesting people, and how badly could it be butchered for television? It was only PG back then.
Showdown is one of those movies you should see just so you can say something like, "Hey, I saw this old western with Rock Hudson and Dean Martin, and it wasn't too bad." Of course, if your time is just too valuable, then skip it.
But it's a decent excuse to eat some popcorn.