I was one of the beta testers and I think that this is a phenomenal resource.
First, I look at the Guide as a catalog of the books at MobileRead. Unlike the Feedbooks Guide and others, this well-crafted book contains the book descriptions as provided by the uploader. It's ideal for reading even when I don't have network connectivity (can you say "airplane") and planning what books I might want to download at the opportune time.
You wouldn't have to be a Kindle user to take advantage of it. Yes, Kindle users can download directly to the device over the Whispernet connection, but the Guide can be used by non-Kindle owners or Kindle owners who cannot connect to Whispernet.
Download the MobiPocket file with your browser and then open the Guide in MobiPocket Reader. If you're reading it on a Windows computer that has network connectivity, you can simply download any of the books using the links in the Guide. Note that MobiPocket Reader V6.0 won't open the book directly (or at least it won't on my computer and I installed it with all the defaults), but you can save it to your hard drive and then open it to read. I don't know if MobiPocket Reader on Blackberries or other networked hand-helds has a similar capability or not.
Which leads to a question whether the Guide would be useful in other formats. How about a Guide for Sony-format books? Even if there is no direct download from the Guide to a Sony device or workstation emulator (calibre?) I can see the same value in having the catalog on your device as I do to reading it on my Kindle.
OK -- I know it's asking a lot of our fearless programmers, but how about it?