They are neat, but from what I've been told you have to keep a second one around for any wood that might still be slightly green, because the false alarms cost you about 100 bucks each for the replacement cartridge.
badideasinaction - costs more than that. 80-100$ for the cartridge, and likely 0 to replace the blade. That said, I wish to hell I was using one of these when I put my left hand through the table saw.
They had that 'saw stop' device on a discovery show a few years ago that did super slow motion video. The inventor put his own hand in the thing to prove it worked. That's when I realized I would never anywhere near the amount of balls that he did.