Old thread...but just in case someone reads this:
Like JDRoberts, I bought one of the kits that had an "old" motion sensor. It works great and has excellent sensitivity.
I just purchased a 5 pack of motion sensors from ST. They are the new ones and they aren't anywhere near as good as the first generation. Even ST Support admits that they are less sensitive and should be placed perpendicular to the path that you want to monitor. That's all well and good if you just want to monitor a doorway, but what if you want to monitor an entire room ? Last night, I sat in a chair that was below, and at an angle, to one of the new sensors. I was waving my arms above my head...no motion sensed. It wasn't until I stood up that it detected motion. Supposedly they have problems detecting straightline motion because, according to ST Support, a moving object must cross 2 lines of detection in order for motion to be registered. In other words, you can walk straight towards one of the new ST motion sensors and it won't detect motion. This was supposed to decrease false positives...but, in my opinion, it decreases TRUE positives.
After exchanging a few emails with the helpful support folks, I'm asking to return the 5 ST motion sensors that I just bought and I will be looking at 3rd party alternatives.