I've tried a number of the cheap models over the year and it's true what they say. You get what you pay for. When I got into Highpower shooting I went out and bought a Kowa. Yes, it's more expensive, but damn, its one clear piece of glass. I could see 223 holes in my target up to 300-400 yards away. It has also been rock solid and has a lifetime warranty.
For something like this I can say spending the money for the Kowa has saved me money from buying a number of the cheaper models to find one that worked.