I have not tested your plugin yet, but I have read through the description, FAQs, etc. I am just glad to finally see some competition to WP-Invoice. It is just nice to have options sometimes, and your plugin sounds like it may become a formidable challenger to WP-Invoice http://wordpress.org/plugins/wp-invoice/
I think that to be on par with WP Invoice, you will need to build in some front-end functionality. It is nice I think, for my clients to be able to come on my site, click to their account, and see what invoices if any, are outstanding.
I did not quite get from the descriptions and other information about your plugin, whether clients can make payments, or be directed to a payment page, when they receive their invoice via email for instance. I think this too, leaves WP Invoice still with an edge that I would not mind seeing challenged.