I have used all of them......well not every model, but I have used low end and high end models of all three. I have not had any grief with any of them, but prefer the Leupold and Nikon out of the three.
There are so many brands of scopes out there now that it is getting hard to keep track of them.
I recently played with a couple of models from Maine Vue Optics in Maine and they all worked well, have lots of good features and are very affordable. No they are not the same as a high end Zeiss or Leuopold but they worked well, adjustments were repeatable, optics clear and stood up to some serious pounding from my .375 H&H.
www.mvoptics.com