The joys of living with more than one cat and watching them interact almost always outweigh the extra expense. And if you have an indoor-only cat, a feline companion can do wonders in staving off the boredom that can result in stress or destructive behavior. Even if two cats never become good friends, it's rare that they don't eventually call a truce. At least they have another living being around for distraction when you're out — and chasing and tussling are good forms of exercise.
If you already have an adult cat, a kitten (or another smaller adult cat) is usually easier to integrate into the household than another full-sized adult. He'll be considered less of a threat by your resident cat. But there are no hard and fast rules. Many people have introduced adult cats with great success. The chances of having the integration work are even greater if the newcomer is already sterilized, is already accustomed to living with other animals and has a personality similar to your cat's. Cats of opposite sexes often get along better, although again, there are countless exceptions to this rule. Don't bring unneutered males together; territorial conflicts may trigger offensive urine spraying.