Anybody using Jay's Cache Frequency Control? - Is it useful?

It reduces depth and sensitivity in all cases. The only advantage I can see is that if rally type events are your thing and you are getting wiped out by other detector users on the same or a close frequency then it will shift your machine slightly off frequency and stop much interference.
The effect of a slight frequency shift can be seen with machines like the XLT. You can move the frequency slightly up or down but any movement from the optimum pre set starts to reduce performance.