How bad is it? It's a spaghetti swords-and-sorcery film, and a bad one at that.
Should you see it? Yes.
One director of this (there were 3) also made other terrible films ("1990: The Bronx Warriors," for example) and it was the fourth Italian fantasy film for Lou Ferrigno, who adds worst Sinbad to his worst Hulk and worst Hercules titles. It has terrible narration and music and laughable effects and all the voices, including Ferrigno's, are dubbed.
The plot has Sinbad and his crew, plus a prince, fighting an evil wizard named Jaffar for a beautiful princess and he has to recover four gems in the process. There's a pit of snakes that willingly get braided into rope. There's a tub of piranhas that are obvious puppets. There's a slow motion fight with a rock monster. There's undead empty suits of armor. There's a prison of light beams (filmed both in forward and backward motion, apparently to pad the film). There are ogres, amazons, a dwarf, a viking, a clone of Sinbad and there's absolutely terrible - and very funny - dialogue, probably written by director Tim Kincaid. My favorite moment has Ferrigno throw away a sword in order to fight. Look for female bodybuilder Teagan Clive in a small role.