Mammillaria gracilis is an inoffensive little cactus. When it's young, the spines are soft and curve down, so you'll only get poked if you really, really want to. As it ages, though, it puts long, sharp spines up through the top in a regular pattern. It puts out lots of tiny little pups all over it which can be knocked of with the slightest touch, and will grow easily even if you don't push them into the soil. Since they're so small and easy to handle, I use them at the bases of potted column cactus to fill in the pots. Since they're so easy to grow, I now have upteen galillions.
It looks and grows best in 6-8 hours of sun, from morning until mid-afternoon. If it gets full sun all day, the bright white color tends to turn off-white.
Mammillaria gracilis has lots of tiny flowers in late spring until the fall. If it's grown with too little light, there will be few, if any flowers, but they still look good.
These pictures are not really representative of the species. They were taken in 2005 when I wasn't really paying much attention to them and just used them for filler. They grew and aged in less than ideal lighting, making them elongate as they reached for sun. Stress can make plants do atypical things; as you can see, these are trying to flower even though it is December.