I normally work in areas where mobile/cell signals a poor. I therefore have to keep a separate Timesheet and invoice when I am back in the office. I find this incredibly irritating, because invoices done later always take a lot longer. My own accounting system based on spreadsheets did not have this restriction. Xero may in the long run save my bookkeeper some time, but for me, the person at the coal face, it is quite a lot less efficient because of this inability to do invoices on customer sites.