My cat came with them installed when I got him, and I've kept them on him ever since. He doesn't mind them at all - the ones I use are the full-claw-cover kind. They work really well.
It should be noted that if one has a feral cat problem in the area, it is because there is an abundance of food in the area - usually rats, mice, or other such vermin. Cats won't stay where there is no food - this is something that the idiots in Europe's Middle Ages didn't figure out. When they decided to start running around killing off "random" cats, the mice and other vermin multiplied out of control, thus spreading the Black Plague. Millions of people died. It took Europe hundreds of years to return to the population and civilization levels that existed prior to the cat exterminations and the resulting plague. Don't think it can happen again? Two people in NYC were diagnosed with the Black Plague in 2002.
Moral Of The Story: Don't kill cats. You might just be killing yourself.
Cats stay around when they are being fed. A well fed cat will kill for fun.