I'm using this plugin for a few years now, and the developers continuously add new features to a robust plugin. Yes, setting it up takes a little careful thinking but when you consider that this plugin deals with nothing less than your invoicing and their payments, it makes sense. It supports recurring (i.e. cyclical) invoicing and works very well with the separate CRM plugin to deal with customer relations (communication) management, and can take care of customizable automated email notifications. There are some useful (paid) extensions available such as PDF versions of the invoice that you can then have your clients download themselves, or you can email it to them separately, or single page checkout on PayPal (which necessarily requires PP Pro and an installed SSL certificate + static IP address).
Considering how complex invoicing is, I am grateful for this wonderful solution.