The interface is very user friendly, a bit complicated for the first glance, but just take a time to get to know it. Everything is categorized, you can add items, services, customers, then it makes it so easy to create an invoice or receipt. What i liked also, is that they offer some good-looking templates, i guess this can make your company stand out, while other companies often have really simple black&white templates.
Nemesis Kate about Invoice Manager: Create, Send Invoice and Estimate, v3.4.3