Photon just loaded 839 JPEG image previews for me in under 2 seconds. That's expletive worthy speed. And when it loads the full image, it's just as fast, even with RAW format images. And it loads every pixel of a RAW image. If you set the preview mode to fit-to-screen, clicking the image gives you a loupe-style zoom to full resolution. It reads photos–from folders or directly from cameras/memory cards–with a feature they call "Instant Import". I plugged in a camera and by the time I had looked back at the screen, it had recorded all of the images on the card.
Photon serves one purpose, but serves it well. It helps you do a rough cut of large batches of images before heading into Lightroom or Aperture for more advanced procedures. It lacks any form of image flagging, has limited viewing of metadata fields (the columns in the HUD aren't individually expandable or adjustable) and no search feature of any kind. It basically provides a very fast, very streamlined interface for creating collections ("stacks") of images and–once you've bought the full version–exporting them with options to convert the format to jpg, png, psd and more. It does provide histograms, which is handy for deciding on which RAW images you're planning to keep.