I bought a used D610 three weekends ago with the MBD-14 grip and battery holder. SUPER good performance on the battery! I shot 8 straight hours, 884 RAW+JPEG shots, had 28% battery left after the whole day, prior weekend I shot 1,084 frames, during-shoot I did less chimping, but 45 mins' worth of chimping at day's end--with more battery capacity left at day's end!
The D610 creates SUPER-quality 24-megapixel images, and a pretty good in-body sensor cleaning system. Image quality is better than the D3x I had for three years, especially at ISO 320,400,500,600, and higher.
The D810 at $2k is a lot of camera!