I'm using FA for a service business (software) and simply point all refernces to COGS accounts and their counterparts to a suspense account, and allow negative inventory. It all seems to work fine.
Using item category you can define 'units' for your invoices (For example, 'Period' for maintenance services) and set up each service or product sold as a dummy item, whether this is for a software licence or implementation services. These then can drive the accounting entries to the relevamt accounts.
As yet, I haven't hit any real snags in its use for a service business.
We plan to use 3x copies of FA in different countries (with different base currencies) and consolidate in a single base currency.