First of all, just about every aspect of this show's premise was implausible: that Lucy and Ricky would even be married to each other and that they would be best friends with the much older Mertzes. (Although William Frawley was 64 when the show started, Vivian Vance was 42, close to Lucille Ball's age, but Ethel was supposed to be much older). Anyway, the show worked in spite of, or maybe even because of this.
Almost every episode was good, except for some weak ones during the first season. And even though the show is over 50 years old, it doesn't seem dated like other old shows. It's still fresh and entertaining, even after repeated viewings.
There are many well-known stories that have to do with Lucille Ball's vanity. She didn't want Ethel to wear nice clothes, so as not to overshadow Lucy. Vivian Vance's wardrobe was purchased off-the-rack in department stores, and Lucille Ball's was designed especially for her. And then there's the rumor that Vivian Vance was contractually obligated to be at least 25 pounds heavier than Lucille Ball. When Vivian Vance returned from summer hiatus one season having lost weight, Lucille Ball said, "You're looking a little too good there, Viv, we're going to have to fatten you up real quick."
Not enough credit goes to the underrated Desi Arnaz. He was a brilliant, talented, and very, very funny man, but not in the same exaggerated over-the-top style of Lucille Ball. His facial expressions are priceless, especially when his eyes literally pop out of his head. It's too bad that he was overshadowed by his more famous wife. Even his daughter, Lucie Arnaz has been known to say that "my mother gets all the credit, but my father was the brains behind the show."