For starters, it's not business yet - there's no money coming in, except funding. Second, they will do very well as long as Google tolerate them messing with GMail UI. So, it's a feature - yes, a clever hack - yes, but not really a sustainable business.
Insynchq has desktop clients for Windows, Mac and iPhone, and a client API. It can be used for document synching for things other than Google Docs, you can use it for asset management if you're a graphics designer, iterating through versions and allowing a client to see changes immediately. You can use it for synching and sharing any other documents as well.
How is that a mere hack?
I think it's due to your branding/feature page. The way it's presented, it's easy to think it's only work with Google Docs.
It doesn't just breaks GMail UI, but it hides the precious ADS panel.
Unfortunately I (and everyone concerned about privacy) won't let anyone beyond google "reading" my inbox.