No, it's not completely free. You will have to register, but at least you can download the app and start using it right away.
The big downside is the app's reliance on a single database (which, in turn, can potentially cause issues to your invoice list if you decide to work remotely). However, even with these (and we stress the "even" limitations, Manta is a great app that's well worth your time and money if invoicing is part of your 50e0806aeb javamars