A Swiss Ball (fitness ball) is just another tool in the arsenal. It's neither perfect nor useless....it's all about what you do with it.
I bought one for my mom, though, so obviously I feel it must have some value
It's all about the intensity and frequency of your workouts, along with the other aspects of your lifestyle, that will determine the success of your fitness regimen. So if you're doing things "right" on the Swiss ball, then it's effective.