I also run a time tracking service (http://www.spiketime.net <- sorry, I had to ;-)) and just tried Tiempo.

I really like the idea of instant invoices and the approval process (just the fee is a little bit high, I think).

Some random thoughts: - What's the difference between an employee and contractor - is it just the label or do they have different rights in your app?

- When you start the mobile app for the first time without using the website before you don't know the meaning of the icons for draft, submitted and approved (I thought the icon for approved was the save-button).

- When using the timer I can't select a customer/service when the timer runs as you do this when you click stop timer. Also it isn't possible to make any notes on a running timer