I'm not in the market for a new invoicing app, but if I were, the perceived sustainability of your business model is a dealbreaker for using you for it. My business deals with a 6 year audit window. (Technically speaking, multiple overlapping 6 year audit windows.) If your business fails, and I get audited, bad stuff happens. (e.g. The National Tax Agency might come to the conclusion "Mr. McKenzie, you claimed you had $5,000 withheld by clients in 2010 on your tax return for that fiscal year, but you seem to be unable to support that with original documentation. That's unfortunate, but we believe you're acting without malice, so in lieu of imprisoning you we'll just ask you to pay the $5,000 plus statutory interest of 8% a year."

Does this adequately explain why Free-as-in-Beer invoicing software doesn't exactly light a fire in my belly?

Will you be publishing ads on invoices? What is your plan to have the service be sustainable into the future?

I respect your desire to want to office the service for free, but I don't think your comparison will help to assuage anyone's fears of you disappearing.

Without some obvious sign of how the service intends to fund itself a lot of people will walk away assuming that it is advertising and/or mining (which they won't like for their invoices), that it will go away as soon as some unknown reserve runs out, or that it will suddenly become expensive once they are dependent on it. They won't believe "we never intend to charge" (they've heard that before and it hasn't always been true) and won't email you to ask (they just head off on their merry way).

Also the live PDF view, which clever, is really also annoying as any typing or mouse hovering makes that area flash.

You could charge for extra services, for example API access or "Sell my invoice"-integration.