Oh, tell me about it! I like to use tags to organize by fandom, and while I'm not, you know, the most polyfannish of fans, I read enough fandoms that using the built-in system to organize into "Shelves" is a serious time-consuming awkward pain-in-the-ass. It's almost making me think that I should buy a third ereader (I also have a Kindle DX, a gift, that I use for reading PDFs) that's Calibre-tag-friendly for AO3 fanfic. You know, with my infinite spare change. :-P