My Mum had one a few years ago at the age of 90, and felt no pain at all. The op was a complete success, and they only kept her in overnight because the op was so late in the day and because of her age. She was right as rain the following morning and had no side effects.
I had a stent put in after a mild heart attack in '09 and the whole procedure was painless. Some people will experience mild pain and/or discomfort and, as Canary says, whether you get kept in overnight depends upon the time of day of the op. This is because a small amount of bleeding occurs afterwards, which they 'plug' and only when they are totally satisfied that it has stopped are you discharged.