I tested D2X mirror lock up function. I find that mirror lock up is very useful.
You can see the difference in digital display of microwave.
Dot of colon in image without mirror lock up is elongated vertically. With mirror lock up, dot is circle.

Experiment Condition:
f=340mm F5.6 1/3sec
Camera:Nikon D2X
Lens:AF-S VR Nikkor 70-200mm F2.8 + 1.7x teleconverter
Tripod:Gitzo G1325 with Really Right Stuff BH-55 Ball Head
Shutter release method:Cable Release
Condition:Room light only