Does this service perform some sort of automatic notifications/reminders to clients to pay up? I could see that maybe being useful. Often times the problem I have is after submitting an invoice and needing to follow up to receive a payment. Often those emails are stock messages that could easily be automated at set intervals so I don't have to think about it myself. Not sure I'd pay $100/m for it though.
For this price-point I'd expect something like Basecamp in terms of features and polish, not something that holds my money for me and skims off the top.