I recently make a impulse purchase when I saw Leisure Arts craft books 50% off at Hancock Fabrics.... consequently, Tattoo Tees made it into my cart. Basically this is a booklet of 25 iron on transfers - here's the results:
This is my before and after picture and my test run - basically, gray doesn't work with this transfers. They are designed to look vintage so they start off soft and fade slightly after wearing:

My second attempt - awesome. Too bad this shirt is for my daughter (and consequently doesn't fit me), otherwise, I'd swipe this one for myself. You can see in her before and after that her image didn't fade that much:

I plan on making another one for myself (in white since it seems to work the best), but here's what I learned from using these:
Start with a light colored (not gray) shirt. The designs are made to give your tee a vintage feel and will start off with a soft look and fade slightly as you wear it so color choice is important.
Pin, pin, pin. The transfers in Tattoo Tees reminds me of the kind of "tattoos" you used to get on gum wrappers as a kid (where you wet your hand and stick the paper to your skin to get the image to come off) - if you barely move it, the image slides. The more you pin, the less likely you will have shifting and the crisper the lines on your finished design.
Protect your work surface (and the other side of your shirt!). There is some ink 'leakage' when you apply heat to your design.
The hotter the iron, the darker the transfer - so press, press, press!
If you're going to put the image on the side of the shirt, offset it slightly to the front. Centering the design makes it so you can't see the image on the front or the back very well!