I guess I have had good luck with my PR511's.
The only problem I find, is that the lens on the motion detector "rots" away. I think it is not UV proof, and just ambient sun light will slowly deteriorate it. Because of this, I have eventually had to replace mine.
I do know that there seems to be a long lag after making settings, and for the unit to become active. It needs to settle down time or something.
One thing to remember is that start unit numbers wrap.