As a photographer here in the Napa Valley, I am tasked to shoot a lot of events. Unfortunately, very few of them are outside in beautiful light. Most are inside wine caves, barrel rooms, anything that's dark basically. And event planners love lighting everything with candles. So, I have a choice, use ugly on camera flash or go au natural. Depending on the exact situation, I'll use one or the other.
When Nikon announced the D5 with 102,400 ISO, I knew it was time to invest in that beast of a camera. I sold my Nikon D800E to partially offset the cost of the new camera. Now, I have a Nikon D810, D7200, and the D5. I love them all for different reasons. The D5 is perfect for event photography with its relatively small file sizes (20.8 MP), high ISO capability, fast focusing in the dark, and controls that light up. The fast shooting rate (up to 14 images/second) comes in handy on the rare occasion I need that speed. It focusses accurately and quickly even in really low light.
The only issue I've had with it is the occasional inability to auto focus. When using my Nikon 85 1.4 and Sigma 20 1.4, the camera may stop auto focussing. I've learned to turn off the camera, remove the lens, reattach the lens, then turn the camera back on. That seems to solve the issue, but I will probably be contacting Nikon about a replacement.
You can read other testing sites to get more detailed information, but my real-world review of the camera is extremely positive.
Here's an image taken at 102,400 ISO: