I had one.
I had a history of complete dislocations of the kneecap... each dislocation was excruciating, with the right kneecap all the way out on the right side of my leg, waiting for me to pop it back in.
The killer was, each occurance made another more likely. I was damaging my cartilage in the process, and could feel (and hear) grinding in my knee.
Three years ago, after a season in which I had to ice the knee after every ride, it popped out one last time, during a simple light lift.
My doctor explained that the lateral release was my last resort for relief from the dislocations, so I had the surgery later that year, over the X-mas holidays.
I was on the Lifecycle at my gym 3 weeks later, and on my own bike, mounted on the trainer, after 30 days.
I trained for and rode four centuries that summer, with no pain. It took two full seasons for my leg to return to it's former size, but it had returned to it's full strength over one season.
A few weeks ago, I completed the RAMROD, a one-day, 154 mile loop around Mt. Rainier, Washington State, USA. The ride crossed two mountain passes, totalling 10,000 feet elevation gain. I think I've fully recovered.
I don't even think about it anymore, and I'm glad I did it.
Steve