I've never seen a 'bad' one as some people say. and they have all been skirmishable.
The cheaper ones tend to ghost a little bit, especially when you whack up the power in daylight, then forget to dim it for indoors/shade.
my advice would be:
Buy a cheap one from a UK retailer, that way you get the cheap clone without worrying about returns issues if it IS dodgy.
Avoid the Strike Systems 553 (around £100) for sure, not had a look at any others, but that model was an utter pig when we last had them in.
The other thing to remember is that there are a few different reticules around, pick one that you like:
circle with a dot in the centre
Circle with dot and horizontal and verticle intersecting lines on the outer cirlce.
both of the above, but with 3/4 elevation dots below the central dot (what I tend to use)
Zombie stopper eotech with biohazard symbol for a reticule (I believe this is also on CoD?)