The EOS RT has the normal mode and RT mode. With the switch set to "RT" it will fire the shutter with the 8 millisecond delay (from memory). A part of the reason it can do this so fast is it will also stop down the aperture when you focus (as soon as the autofocus locks the mechanism stops the lens) and not when you release the shutter. So, if you set the camera to RT mode and focus, the camera has stopped down in advance, the mirror doesn't move--all that happens when you depress the release the rest of the way is the shutter opens and closes. I haven't tried the EOS1-N RS yet, but this would be even better, with many more features. Both RT and 1-N RS were available quite inexpensively from KEH last time I checked, although personally I don't need autofocus very often, so the RT gets little use these days.
But there are certain applications where they are almost unbeatable. I think I may have mentioned once that I took a few fireworks shots three years or so ago. Had an RT loaded with colour neg and an EOS 630 (on which the RT is based) with Provia 100F. It was so much easier to make long exposures with the RT, because you could actually verify that you'd captured the bursts during the exposure and could monitor the framing. With the 630, it was a case of hold the cable release open and hoping for the best... For flash work (as was previously mentioned) you get the benefit of watching the flash fire during the exposure, like a TLR or RF, which can be useful sometimes, too.
The RT/RS mirror bounces 2/3 of a stop of light up to the finder as I recall. So ISO 100 film effectively becomes 64. A friend has a Pellix (as well as an RT). The Pellix was sophisticated for its day, but the viewfinders are chalk and cheese. The RT finder is a lot brighter. And the mirrors have held up well to date and seem to be more robust--I have cleaned mine (carefully) a couple of times with no trouble whatsoever.
Regards,
Brett