We work with businesses in QuickBooks very often. We can't even get them to switch out of the crazy processes they've created because they didn't understand the program when they set it up, much less to another platform.
You should be afraid. Everyone uses it differently, there are lots of opaque quirks to development and Intuit does wild things like kill their support forums without leaving the old content up for posterity's sake.
There are 2 or 3 different APIs, with verifying levels of support for various features. At times QB will push a release that breaks everything for all your clients and you're left to deal with it.
I wish I could +1 this a million times. I develop a Ruby gem for the QB API (Online & Desktop)[1] and I've come to realize their whole API is a mess. I've also worked with the Xero API and its much simpler but still lacks some key features like adding a discount to a line item on an invoice.
