I started with 6 and upgraded to 7. I had a few bugs at first, but received a 'fix disc' from Apogee. It has worked fine from there. Yes, I hate the cluster problem, but I am sure that this will be offered as a free upgrade.
I use Rocsim all the time. My main interest is in scale models. Since scale mod-rocs do not have the gyroscopes and gymballing motors of the full sized protorypes, many times, the model is actually unstable. Rocsim allows me to play with the design and determine if I want to add nose weight (including exactly how much) or expand the fins a bit, etc. I love being able to do this.
Basically, Rocsim allows me to build models that are 'close to the edge' and ensure that they are safe to launch. In the past I had to build a boilerplate model, and string test it, then play with fin size, or nose weight.