How bad is it? Well, it's a Spanish western with William Shatner in a dual role. Bad enough.
Should you see it? Don't go out of your way, but if it's easily available, give it a try.
While "Star Trek" was on hiatus, William Shatner went to Spain to star in a western, perhaps hoping to do what Eastwood was doing in Italy, but more likely just trying to get a paycheck. He plays twins, half-breed Native American, distinguishable only by the color of their contact lenses, er... eyes. One of the brothers believes he is the one to lead the Indians to victory over the White Men, partly due to the amount of peyote he takes - drug references were "hip" in 1968. The other brother lives as an outcast among the townspeople, who gets called on by sheriff Joseph Cotton to fight his brother. So... conflict among the natives, conflict among the settlers, conflict among the brothers. Of course, they do the old Prince/Pauper switcheroo. There's an odd jazz-influenced score, there's anachronisms (telephone poles, for example) and there's some mighty fine shooting, as every bullet goes right between the eyes.