Die hard fans ripped this one apart, and I heard somewhere that Leonard Nimoy decried it as the first of the "monster episodes" and how that wasn't what they'd wanted for Star Trek to be. However, this episode has too much fun stuff in it for me to give it a poor rating. Most notably, in our household this was one of the most "quoted" episodes as far as cheesy-fantastic lines. There's ditzy Janice Rand's glib take on Green to Sulu, "Do you think he's space happy?" and Kirk's smirking and scoffing McCoy's reaction to his old flame, right in front of the apparent husband, "No offense...she's a handsome woman...but hardly twenty-five." Then there's Spock, belting the incarnate Nancy back and forth across the face, "If she were Nancy, could she take this?!" and McCoy watching in mild puzzlement. It really seems like he's about to say, "Well, Spock, Nancy always was a tough ol' broad..."
Other highlights include Kirk screaming like a girl when the creature closes in on him, and the deliciously hideous monster itself, when in its true form.
All in all, it's full of goofs and question marks, but I rate it a "good" for pure entertainment value.