I have the RX-III. I haven't had any issues with mine. You need to spend some time understanding how to run the thing before you go into the field. I have found that as you start ranging things out farther and farther you have to have a good rest to get consistant readings. Just holding it standing up while ranging out there 500+ yards, the slightest shacking makes your target area pretty big.
One guy mentioned he had trouble getting it to read on his bow target. I don't know if your target is black or not, if it is, its not the range finders fault. Black does not reflect light, so you won't get any light coming back to your finder to measure a distance. Try putting a piece of paper on your target or spray painting it some color.
The reason all these range finders have such a short warrenty is because they are rely on electronics. You won't find any electronic item that gives you much of a warrenty.