Some of it depends on how you use AC, some of it depends on how ready you are to upgrade software/hardware, the last part is what specific functionality you need/want.
We have had one client who had a catastrophic failure with the upgrade, and we did a hard roll-back to 6.3.3. At the same time AC could not provide the Friday backup on the following Monday. They have sense used a forensic instance to demonstrate problems to AC with billing data after the upgrade. They also found the newer version slower.
We have another client that is running 6.5.x without using the billing, and they are alright, but don't want to do any upgrades until we give the all-clear.
For most practices, they are better to hold off, and ensure that they are storing their own backups.
