I hadn't seen that before. Time tracking is probably the harder part to make a polished solution. Invoicing is much less frequent, and while I'm using to a desktop app now, I could live with a web app.
Time tracking though - if it's not a button in my menu bar I can click start on (and helpful stuff like noticing inactivity and pausing/prompting etc) I'm going to forget to enter times.
I've wondered sometimes if it'd be possible to fork the toggl desktop app (its open source) and reverse-engineer a backend to support it (or just update the desktop app with new backend calls).
https://github.com/DefactoSoftware/Hours
Then the only thing that would remain is to take the hours registered in Hours and stuff them into invoices somehow...