This is the most interesting fish to swim into this crowded pool in a long time. It has all the essential features for efficient job timing, but with none of the redundancy (invoicing etc) that is simply bloat if you don't need it.
myMacTime differs from most of the advanced work timers by keeping a separate document for each project. This has advantages and disadvantages. It makes the interface simpler in some ways, enables you to store the timing data in Finder project folders (or wherever you like), and makes concurrent timings across projects possible. Sessions and tasks can be organised and reorganised very easily in hierarchical lists in the LH pane of each project window. It does not however (yet) provide a centralised interface from which to control all your project timings-unless they are still in the Recents submenu.
The interface is very intuitive and Mac-like, most things can be controlled from the (optional) status item or using customisable shortcuts, and there is a useful option for the project windows to auto-hide, so that the app stays out of your way most of the time. There is even support for stopping and starting timings using the Apple remote.
There is exemplary implementation of drag and drop, within projects, between projects and even to external apps. One or two things are missing in version 1: the status item does not indicate whether or not a timing is running (though the dock icon does), iCal synch is only one-way (export from mMT) and there is no provision for adding expenses to the lists, but the developer is extremely responsive, and plans to add all these features, and more, in coming versions.
All in all, very well thought out and well designed app with a promising future.