There are a couple things about writing my Ruby wrapper for the C API that make it particularly fun, mostly related to getting to add abilities that I desperately wish were there in the normal APIs.

1. Ruby-style (forall) looping. Anyone who has iterated over a NotesDocumentCollection knows the drill: set a variable to the first element, start a while loop, and make sure to set the variable to the next one at the end. Writing it one time isn't so bad. Writing it hundreds of times, though? It gets to be a drag. Getting to write docs.each { |doc| ... } is a breath of fresh air.
2. Easier design-elements-as-notes access. The normal API lets you get a NotesDocument version of a NotesView via its UniversalID property, but for everything else you need a NotesNoteCollection, which is a hassle. Since all design elements are Documents anyway, I've just made their wrapper objects subclasses of Document (though I may change that to just a #document method if it gets hairy) and I've put a #get_design_note method on Database that lets you find a design note by name and flag class (NIFFindDesignNoteExt).
3. HTML and DXL everywhere. I use DXL fairly constantly (mostly for design elements), and for the most part it's a hassle. Not only do you have to create a NotesDXLExporter, but you also have to get the Document version of the design note you're dealing with and run through its process. Not impossible, but sort of a hassle. Now, though, I just put a #to_dxl method on everything - this is like the .generateXML() method in Java, but consistently applied. Similarly, I can't count the number of times when it would have been handy to get an HTML representation of some element, even if it was just the dated stuff that the legacy renderer puts out. Since that's all there in the C API, I just put a #to_html method on everything and a #get_item_html method on Document for very easy access to web-friendly versions of MIME and Rich Text items.

It's just a shame that I probably won't be able to use this: what I'd really want to do would be to use it like a database driver on a Ruby-driven web site, but the fact that it's so tied to ID files makes that tough. Still, it's great exercise to write it, and maybe I'll cave and set up some sort of multi-process hydra beast to suit my needs.