I've seen some very positive reviews on the Zen-Ray ED2 82mm spotter, claiming it is "nipping at the heels" of the alpha spotters (Leica, Swaro, etc.) although I have no personal experience with it. Read about it on birding forums, some of those guys are nuts about optics.
That said, spotting hits on shoot 'n see targets at 600 yards with even the best spotters made (Kowa Prominar 88 or Swaro 95 modular) might work under the best conditions, but it will be far from reliable.
You need a target camera system to reliably spot at that distance and beyond. I like the system from targetcameras.net; a complete system that uses RF, not Wifi, to operate up to several thousand yards. When I do finally break down and buy a target camera, that's what I'll be buying.
Cameraland NY is blowing out their ZR ED2s for $750 shipped right now if you are determined to buy one. That's really cheap.
Joe