I wish i had bought one a long time ago, it has been very helpful. I got a baby scale that weighs in pounds and ounces (or metric, it has a switch), it's much more accurate than the technique of weighing yourself on a scale intended for adult humans and then weighing yourself again holding the cat which usually only gets you a ballpark figure. I originally got the scale so that i could monitor Rajah's weight as she recovered from her anorexia, but found that Elvirah was also slowly gaining weight that she didn't need, but she didn't look like she had gained any. With this info i was able to balance the portion size so that Rajah slowly gains, and Elvirah's gains have stabilized and then slowly reversed.
Unfortunately they are both completely on dry now while i sort out Rajah's allergy issues, but once i get the dry under control i'll introduce wet until i find one that Raj likes and can tolerate, at which point i'll have to rebalance their portion sizes, and the scale will give me quick feedback to see what adjustments i need to make to keep them both on track.
I bought the scale new and it was worth every penny. Mine has a great "hold" feature that averages the weights over a brief period of time to compensate for babies (and cats!) that won't hold completely still for weighing. After each weigh-in i write down the date, their weight, and note whenever i have had to switch their food, etc. Having this record is extremely helpful.