Fantastic app. It's called GeekTool for a reason; you have to already have some geekery ability to make it do much of interest. If you don't know what a shell script is, or how to write one, or are totally unfamiliar with things that live in /usr/bin, you won't be able to do much besides put images on your desktop. And most other things don't make a lot of sense then. But if you are a Geek, it couldn't really much get easier to use.
It could use improvement in certain areas. For instance, it often reshuffles the order of which geeklets get loaded first. This can ruin a carefully-crafted desktop. If you try to fix this, you can, but you run into the fact that GeekTool keeps track of its geeklets by means of hexadecimal UIDs instead of the names that you already gave your geeklets when you created them. So to shuffle their order in the plist, you have to write down which UID goes with which geeklet. There should be a box or something in the app's preferences that allow you to change the load order of geeklets, and shows you a list of them by name (not UIDs).
It would probably work smoother if there was something in the preferences that allowed the user to set a custom $PATH.
One gripe: 3.1.1 came out, and it destroyed the look of Monaco at 9pt and 10pt. They are now antialiased, where they never were before. Monaco looks *terrible* antialiased at those sizes, and having it antialiased makes things displayed look much, much less geeky :) Indeed, I'd love to see it get the ability to optionally disable antialiasing for *any* font (like in Terminal).