I'm an IT consultant in the UK who has already more than covered the cost of this excellent software by streamlining my invoicing process.
What would be invaluable to me would be the ability to have separate invoice number sequences by client. If I had a client called Acme Pharmaceuticals, and allocated them the code AP, it would be perfect for their first invoice to be AP001, the next month's to be AP002 etc. This would be particularly useful so that a client couldn't work out how many other clients I have by looking at the number of invoices I've raised between their last two.
I don't know how trivial this would be to offer as an option, or how useful it might be to your users in general, but it would make me very happy.
Also, I found that I'd forgotten to apply a non-standard rate to a new client, so the default was used for dozens of entries; I was surprised to see that changing the client's rate could not be applied to existing entries. I understand why this is sensible (on agreeing a rate increase, for example, so that prior entries were charged correctly), but wondered if retroactive rate changes could be added as an option?
Thanks again for the best software investment I've made on the iPhone.
Darren