Glenn,
I have two of these on my layout. I bought one upper quadrant and one lower quandrant semaphore signal. They look great and operate very nicely. I also have numerous other signals by Z-stuff on my layout, PRR position light, and the regular block signal (3 color light).
I too like the benefit of having the built in detector.
The only down side I have experienced was that my one semaphore signal was damaged by a derailment once and has not really operated properly since. The signals are plastic rather than diecast and are easily knocked down if hit by anything. But this would not stop me from adding more of these great signals. I rarely experience derailing trains in O scale, so not really a major concern with signals regularly getting hit.
The biggest problem is finding the number of signals you might want to buy. I found a few at Walthers, a few at a store locally, but seems the production is sporatic and you better buy these when you see them.
Scott