We have enhanced Dynamics GP Field Service to provide PSA functionality for IT companies.
Take a look at our quick video.
We use built-in Dynamics GP Field Service but extend it using the (free) GP Webservices connected GP Time Tracker module. Adding time to a GP Field Service call is 2 clicks! Wow! Use it inside and outside your network via webservices!
Since Dynamics GP Field Service module is tightly integrated there is no messy inventory, customer and invoicing syncing issues to deal with.
Let me know what you think.