It's sort of a pair, but the Ovilus II and the PX. I didn't realize until I read Nick's book that these are both Bill Chappell devices, so that's wicked in and of itself. Bill comes up with some awesome things. :-) These also gave me a bit of a hint that Zak and I are close into the same age because, when I first heard about the Ovillus, I said it was like a Speak 'n Spell (which my brother and I both loved). Zak describes one of these as a spirit Speak 'n Spell.
Anyway, to the why part of the question. I like how easy they are to understand when the spirits are using them. I really like the screen on the PX device that shows the word that the spirits just said. I think it adds to the accuracy of what's going on, and can help bring up other questions to ask at that location.