This seems like a very good start.
I like the flexible data entry, like the ability to type dates in yyyy/mm/dd or mm/dd/yyyy format in the date fields, the information in the starting points menu, and the way the developer seems to have put a lot of thought into how to make things simple. Controls are located right where you would expect them. The program has a very simple and well-organized interface, and is pretty easy to use for the most part.
There are a few things which would really help:
The lack of right-click ability is annoying. In the left pane, I find myself right-clicking on "Clients" hoping for "Add Client", right-clicking on individual clients hoping for "Edit Client", clicking on individual clients or other items and hoping for "Delete", or clicking on the "Services" category under "Clients" hoping for "New Service".
In the top pane, I find myself right-clicking hoping for "Copy"(to copy an entire item, or if an individual field is selected, just the field), "Delete" and "Duplicate".
In many Mac programs, like in the Finder, when something is selected, you can hit return to highlight its name and rename it. This ability, and the ability to hit tab and shift-tab to get to the previous or next item would be useful both in the left pane, renaming clients or categories, and in the top pane, quickly changing client, date or summary information.
A delete button would be a useful button in the button bar at the top, and the ability to go to Edit:Undo to un-delete an item from the top pane would be much appreciated.
"+ -" menus at the bottom of the left pane like the ones at the bottom of the client editor would make it easier to add clients or folders to sort things.
The invoices look very nice, but I'd also like a little more ability to edit what appears and the location of things on the invoice.