After reading a post over at fitsugar.com on if running every day is safe and okay, I wanted to make some comments.
I use to run nearly everyday back when I was on cross country and track in high school. It surely has its disadvantages and its advantages.
The advantages include that working out is healthy for your body and can help you lose weight and even have more energy throughout the day.
However some disadvantages are that it can cause injury such as tendonitis and wear down your joints.
If you are young and training for cross country, track, or some other event such as a marathon, I think it is perfectly okay to run 6 days a week regularily. However you should make sure you mix up your routine and change your millage. For example, perhaps one day you run 8 miles, the next day you do intervals where you run for say 1 min fast, then 1 min jog, than 2 min fast, with 2 jog, and then 1 min fast, and 1 min jog. Of course vary this more or less as you see fit. The next day perhaps you take it easy and only run 3 or 4 miles. The following day you could run up hills and then jog back down to the bottom and run back up the hill.
I personally no longer run more than 2 or 3 times a week due to a knee injury that occurred a few weeks after I had my wisdom teeth removed. (Read more on the homepage for my story). However nowadays I often swim and bike on the days I do not run. It is advisable to mix up your workouts and not always exercise the same muscles. Weight training is also a very beneficial.