To get any benefit from the letter writer, you have to invest some time to setup templates. It functions like the M$ Word mail-merge functionality.
Something to be aware of is that they functioning of the daily collections report and other aspects of the billing in AC has changed. According to AC, it was working wrong before, but now is working right. Of course, that means the ways you have learned to use AC may not work now because they were wrong.
A client who was devastated by the upgrade and did a hard roll-back was able to demonstrate a bug in the 6.6.1 and 6.6.2 releases having to do with adjusting entries - that bug has been passed to AC Dev to be fixed.
Users on the new versions report the same or slower performance, we haven't heard any clients reporting faster.
Because there is no easy way to roll-back, most practices are waiting to see a version that is stable and has functionality that makes the cost and risk of upgrading worthwhile.
