I think that J had the potential for a great villain. She had a dark personality that made people hate her for good reason, making her one of the truly evil people in the anime. In addition, she used very unconventional yet effective methods such as targeting trainers and using her goons as expendable resources. This made her a very challenging opponent.
That being said, I feel that J's potential was wasted in the series. Her cold view on people and Pokemon would be an interesting contrast with the heroes, even moreso than Paul; however, the directors failed to give this enough emphasis. Also, while her end was dramatic, I felt that she should have perished fighting the twerps, whose strategies and values are a world apart from hers. I was hoping that going into the Lake Trio's void would give her some power that would allow for a final battle against the party. Sadly, the writers apparently wrote her off. It reminds me of Charon; they portray him as an imminent Starscream and let him escape capture, yet never bring him up again.