Ball diffs are a good thing to have. They change the way the car drives , especially on slippery surfaces or with powerfull motors. Yes, they are ajustable but this is often not used once the diff is installed. To adjust, basically you want to run the diff (therefore the setting screw) as loose as possible before the diff slips, meaning the pulley can turn without moving the wheels.
On the downside, ball diffs are not really cheap, and they require maintence depending on the track you are using and how sealed or open your diffs are to dirt and grit. So depending on your car and category, they may not be that important. Dont forget many gear diffs are also adjustable by inserting oil or grease to make them act more like ball diffs, and they are more robust.
I would probably advise you to save the money and spend it on another battery or a better charger, more fuel (if nitro), a high powered steering servo (in any category), some better tyres, and especially more practice time. Tyres alone can save you up to two seconds a lap (considering many variables) where as the diffs may only save fractions of a second. It would probably be more worthwile waiting until the next car you buy, which will probably be a better kit and so include ball diffs anyway.
If you have any more questions, just post it, I'll help you if I can. It would also be better if we knew what kind of car you have, where you drive it and if you have handling or performance problems with it.