Declawing, or onychectomy, is almost never medically or behaviorally necessary, and should never be considered routine or done preemptively. It's the worst sort of cosmetic surgery - done entirely for the convenience and benefit of the cat's owners, and almost always to the detriment of the cat. Although it's still widely believed that declawing is a simple removal of the claws--which most people equate with fingernails--the surgery actually involves full amputation of an entire section of each of the cat's toes. It's the equivalent of having your fingers cut off at the top joint!
To avoid additional time under anesthesia, declawing is often combined with sterilization surgery. Many cats recover from declawing with no or few complications. Other cats recover from declawing with severe pain, bleeding, claw regrowth, loss of feeling in the toes, and infections that may require further surgery.
Shredded newspaper is often recommended as litter box filler right after declawing to protect tender toes and reduce the chance of irritation from clay or clumping litter.
Animal shelters report that a large number of declawed cats are surrendered by their owners because of fear biting and litter-box problems.
Source: Complete Guide to Cat Care, Humane Society of the United States