I have never interfaced it with anything, but we have used GNUCash for many years to run a small business and are pretty happy with it.
When we started up we bought the basic Sage package (our only Windows-based software) and started using it - within two weeks it had infuriated me enough that it was dumped for GNUCash which has steadily continued to improve.
All our day-to-day small business needs are well covered; we keep customer records, do invoicing, reconcile with bank (initially using electronic bank data but soon found it just as easy to do manually).
At the end of the year I just run a few reports to get figures for the various income / expenditure categories required and send these to an accountant who uses them to fill in the necessary returns.
Yes, it would be nice if there was a more user-friendly way of customizing invoice layouts, but really other than that I really can't think of much to complain about - for a few years now it has been very stable indeed.
What exactly does LWN do that GNUCash can't comfortably deal with?
I should say that I handle payroll mostly outside of GNUCash, using a fairly simple LO spreadsheet for payslips; fortunately in the UK the government provides software to handle the payroll taxes which amazingly is available for Linux too.