Yes i have sucessfully used them on many cats including my own one.
Yes you have to clip the tips of the nails off first.
In my own experience they do not cause discomfort/pain and are excellent for scratching problems..one of the longest users of them had an extremely rare pre malignant on going acute skin condition which basically meant the cat (a persian) would have a short life. meanwhile for the years he did have he could never ever be without the effect of strong special steriod injections and soft paws on all feet. if he ever managed to get one or two off, or they came off naturally with the nail growing he was so itchy that he inflicted wounds on himself that would require suturing many a time or the least dressing of wounds. this was an extreme case that most vets would not even see in their professional lifetime.
other times they can be used in situations ie you take the cats with you to the parents house at xmas , you move into new house etc times where a cat will normally initially scratch its territory more, this type of behaviour dies down but in the initial period they can still scratch but the soft paws means it doesnt damage.
My own reasoning for using them on my cat was that he was a stray in desperate need of lodgings, due to a medical complaint he was to be caged for 6 weeks, not even correctly penned or kenneled but caged due to no other lodgings available, i found that wholly unacceptable for such a fine friendly cat so took him to live with me in our brand new flat and obviously as my o/h wasnt such a cat lover as myself i knew had there been 1 incident said cat would have been out the door so i took proactive measures on the "just in case" assumption. This said cat never actually left my care LOL and we became his forever home, his name is fraser by the way but he showed total indifference to having them on (only front feet for behavioural scratching and normally just hind feet for medical scratching)
hope this helps.