According to Wikipedia, here are some of the controversy of Heelys:

As of September 2005, some (US) schools can have Heelys. These shoes are usually allowed but the removable wheels cannot be in place. With the wheels removed, the shoes function just like any other pair of shoes.

Some businesses; especially malls, discount retailers, warehouse stores, and grocery stores; find the shoes to be a problem and a hazard. Some children with the shoes use them in the crowded stores, thus creating a hazard to shoppers, a potential risks for falling and injury, and a problem for insurance in the store, even if they aren’t at fault.

There are also alleged health issues. A few doctors - mostly back, leg, and foot specialists –are concerned about how long-term use of Heelys may affect the developing spine and muscles of children and teens. According to these doctors, the stance that children use to ride the wheels in the back of the shoes makes the spine flex against its natural curve in the lower back. It may also cause pain on the side of the ankle going forward.

In 2006, Heelys were the #1 on the list of “most dangerous toys of 2006″. This is mainly because so many injuries are due to Heelys. However, it has been observed that 97.29% of Heely-induced injuries are caused by people doing one of two things: either attempting a trick they are incapable of, or attempting greater tasks when they are learning to use them. Once you have learned how to use Heelys, and you don’t attempt out-of-reach stunts, you are actually quite safe.