The Nike Free comes in several different models- Runner, Trainer, and Trail. As mentioned before, 10 is a regular shoe and 0 is barefoot. The Trainers come in 7 and 5. The Runners come in 5 and 4. The Trail comes in 5.
The best thing to do is to try to different models and see what feels the most comfortable for you. I have the Trail and I found them more comfortable that the other models I tried, but that's just me.
My only wish is that Nike would make a Free that does not have the typical heel of the sole that is thicker than the front of the shoe. I wish they would make the sole level from front to back so it would be totally level like the ground.
The problem nowadays is that all the fancy shoes with all the fancy cushioning are making people's dysfunctional bodies even weaker more dysfunctional. If you have shin splints, knee pains, etc... when you run it is because there is something wrong with the alignment of your body which is preventing it from properly absorbing shock. When you start adding all that cushioning, you are having the shoe do what your body was supposed to do. Then what happens is that your body ends up depending on it, thus deviating farther from its functional design.
If you want to get absolutely as close to barefoot as possible, then check out the Vibram 5 Fingers shoe. That shoe is like a thin glove for your foot. Personally, I have gotten my body to a point where I can run 100% barefoot on concrete and it doesn't bother my joints at all. I actually ran 1.6 miles barefoot on concrete today. I only wear the Vibram 5 Fingers to protect my skin. If your body is at the functional level to run barefoot, you will feel the usage of muscle is very different from run in "regular" shoes, including the Nike Free.