I did some quick research online, and it received mostly positive reviews. But like always there are people who post warnings and negatives they discovered when they tried it. A few common complaints were that if you use red Kool-Aid it can last too long...Some say that this washes out too quickly while others say they couldn't get theirs out and that they had to grow it out. I'd do some checking before you try it, even if it is a simple and temporary method.
Good tutorials on how to die your hair are very easy to find. I think this post was my favorite though. It explains different methods of using the Kool-Aid dye, gives instructions, and includes a video.
My Little Experiment...
I decided I'd give this a try today and picked up some purple (SUGAR FREE!) Kool-Aid packets at the store. When I was ready, I pulled out a small strip of my hair and ponytailed the rest on the opposite side. It's important that you keep all hair back except that hair you're wanting to dye. The tutorial will tell you this, but I boiled 2 cups of water and when the water had cooled a tad I added the 2 packets. When mixed dipped the hair into the bowl and kept my head there for about 5 minutes. I started with really thin pieces and added more when I liked the results. I took a napkin and dried the excess Kool-Aid and then blow dryed the section of hair. Although the results were more subtle than I was expecting, I was very pleased with it. I am a dirty blond and the color looks like a dark auburn/plumb. I will be definitely be trying this again, and maybe I will use a different method next time (like painting the hair and then wrapping with foil). I've included a picture, the quality isn't terribly great but you should be able to get an idea of how it looks.
If you try this, or have done it before, let me know how it works for you! I'm curious how the results differ with different hair types, methods, and colors. Have fun ~Erin