I set up one of these on the bench, and I was quite impressed with how precise the triggering range can be. It works within about an inch in distance. It would trigger every time at, say 10 inches, but never at around 11.5". I have to think this resolution might work for some applications better than an IR sensor that reflects off the target. The IR boards just reduce the sensitivity of the receiver to change the range, that's a very inexact method of adjusting range. I fooled around with a variety of different materials in front of the ultrasonic sensor, and it was amazingly consistent with any of them.
BTW, still have a few sets of parts all kitted up if someone want's to build one or more of these. 
