I am using 6.5.5 and am pleased. I "side-graded" from V7.0.10. It fixed a couple issues with 7 and had a better letter writer. It does have formulary checking but I have not found that useful (does not work with IL public aid which is most of my practice.) Some of the other "new" features were in V7, so I was use to them.
I think it runs faster. I did do several tweaks to the system so I cannot rule out that they may have had an effect. Overall I am pleased.