I'm biased as I'm related to the creator (he's my brother and co-founder) but this is incredibly easy to use. We've used it at Hipmob to process all our invoices (whenever a customer requests one).
Femi (the creator) is also super responsive so drop him a line if you have feature requests! (fomojola@ideasynthesis.com)
How would this stack up against something like Manager.io (http://www.manager.io/) which provides the same functionality and more for free?
It doesn't really compete with Manager.io. Manager.io looks like a full suite (just judging from reading the site) which isn't what this is for. This is useful if you're a freelancer or consultant, or are in the market for new invoicing software. It definitely won't replace your accounting system.