I have been very anxious to have the improved letter writer... for years. Towards that end, I started beta testing with V6.5. Due to reports of problems with 6.5 and more recent versions, we are still on 6.3.3.
One thing that was clear to me from the beta testing, was that the new letter writer could not be fully put to use "out of the box". It was also not intuitively obvious how to efficiently set it up; the process takes some time. We had a plan for doing so, but never implemented it as we are still using the older product.
Bottom line is that I would second Indy's advice. Just as you will need to devote some time and effort to handling changes in insurance entry in 6.6, you will also need to invest time and thought into preparing your templates so you can use the letter writer. You should probably look at these things in a "sandbox" before doing a full install. That is NOT the view of someone who is using 6.6; just the cautious word of someone who previewed it.
