Hi - good to see some work on this. There was a thread about using bambooinvoice as well but do not know what is happening.
I have been looking around for something to use and as well as the two you mention and bamboo, there is also Argentum, which seems a bit clumsy and buggy, but does have projects and tickets that could be mapped to workspaces and time slices and siwapp, which looks pretty good too.
For my purposes, Simpleinvoices looks best. It seems to be the easiest to enter the invoice data into - can set up items and the unit cost and just add the number of - in my case - hours.
MyClientBase seems to be a bit too slow to make invoices to. You need click through too many pages to add items to it (and the demo on opensourcecms keeps giving me errors). I may install it and take a better look when I have time.
I have not looked at the code or done any more serious evaluation than that. Be interested to hear how you are going with it - try to help out where I can :-)