According to Consumer Reports, the ball is mostly a gimmick. They found that many cheaper vacuums using conventional casters and wheels were just as maneuverable, and often better performers.
According to Dyson, the ball serves two purposes. It supposedly gives more freedom of movement but Consumer Reports has said that while it works, it’s no better than some of their top rated non-ball vacuums. Dyson also claims that because the ball also houses the vacuum’s components, there is a lower center of gravity which makes it more stable.
There are many rumors around the Internet that some carpet manufacturers void the warranty if you use a Dyson. Apparently, the beater bar is deemed too rough on the fibers. Despite the rumors, and the many testimonials from self-described carpet technicians, only one manufacturer specifically names Dyson as a violation to their warranty.