My moggies are both neutered, and happy and healthy. When my friend and myself bought this house in 2001, the previous owners left their cat behind (said they wouldn't be able to catch it). He was an unneutered tom, about 3 years old, covered in abscesses from where he had been unsuccessfully fighting with bigger tom cats. He also had an unfortunate habit of peeing on any bit of plastic that came into the house. Once he'd been treated with stacks of antibiotics, he was neutered. One much happier cat, no more peeing in the wrong place and no more fighting. He is now living out his senior years with my friend and her other cats in a house with a nice big garden.
My Campbell cat picks up enough boyfriends despite being neutered, and I dread to think how many unwanted kittens she would have produced by now.