I have seen one of the 427Rs and it was a beautiful car. It will be a rare car because they make very few of any of the Roush models. You know it is going to perform great and always be an eye catcher.
The price is going to be pretty expensive I would imagine and since it is the top of the line for Roush, the dealership will probably not work with you too much. A friend bought a Stage 3 back in June or July, a month or so before my Shelby came in, and he paid about $3k less than I did for that one. They worked with him on that one because it listed for over $50k I believe.
I wanted a Shelby! I have an '05 Mustang GT that I have very slightly modified and I would not have gotten another sporty car if it had not been a Shelby! I looked at it like the Roush is just another modified Mustang GT and I already have one of those! The thing is, Roush makes a really great modified Mustang GT!
If you decide to go with the 427R, I have no doubt that you will love it as much as I love my Shelby! Well, maybe not quite THAT much!