So far I just use it for my company billing, but it's quite delightful for me: My process is to save the services we provided in CSV files (exported from time tracking in org-mode, but you could use any tool that supports exporting), then I run this over it and it creates PDF invoices, and stores all the data (which I query occasionally, e.g. for quarterly VAT payments).
Technologically it's weird, the invoice templates are written in LaTEX, and the code is in Common Lisp.
I don't think about turning it into a business, but I think I'll open source it once it's a bit less messy. For now, I just focus on implementing the features I need (about 1-2 per quarter now).
I kinda doubt it, but if anyone is interested in command line billing tools, I'd be excited to talk about it. Contact info is in my profile.