I currently use a mixture of several different types of sensors, each with different strengths and weaknesses. I use a bunch of Ademco passive infra-red sensors, which are very good at detecting people in general, but often can be slow to trigger, somewhat erratic, and are not really able to be "aimed". I use retro-reflective photoelectric sensors in several places, but those are actually too precise, missing audience members who are not exactly in the right places. I run one microwave motion detector, which actually senses people through a concrete wall, but also senses crew on the other side of the wall and is extremely difficult to adjust. The diffuse reflective sensor seems to be a great compromise, giving us a reasonably wide detection pattern that is, most importantly, aim-able. Here's a little video I shot showing the new sensor while testing in our shop:
(Sorry about the focus, it's tough to change focus on the fly in video mode on my Nikon D90)
The sensor is rated for 10 feet, but in my little test it worked pretty well up to about 13 feet (depending on the target size).