I wouldn't recommend buying this product for the cacheing feature. Despite DVDIdle's claims that it will increase the lifespans of your DVD drive and DVD discs, in reality it will decrease their lifespans.
Based on their description, this application rips the DVD at the maximum speed supported by your drive and stores the data on your hard drive for later access when you need it. In their example they say a 90-minute DVD you are watching will be read from the drive in only 15 minutes at 6x speed, so that the drive can "rest" for the remaining 75 minutes rather than spinning the entire 90 minutes. At first glance that might appear to be a reasonable argument - the drive is only in use 15 minutes instead of 90 minutes, so you have saved 75 minutes of wear and tear. But a little critical thinking quickly debunks this claimed "savings".
The first two problems that immediately pop into my mind are that in order to use this you must have 4.7 GBytes of free space on your hard drive (not too big a deal these days for most people) and that during the first part of your movie you will be assaulted with the noise of your drive ripping at full speed (not necessarily a trivial noise, as owners of 166S drives can attest).
The more fundamental problem, however, is that the energy required to spin a disc is a non-linear function of speed. The primary force acting against the drive's motor is the air resistance on the spinning disc. Although calculating air resistance as a function of speed is a non-trivial undertaking, a good first approximation for assumed turbulent flow is that the force goes as the square of the speed.
Let's use your new 166S as an example. If you have a single-layer DVD disc it can rip at 16x CAV. That means that the force exerted agains the spinning disc is 256 times as great as at 1x. Force times distance equals energy, so for each revolution of the disc your drive has to supply 256 times the energy at 16x as at 1x. And whether or not you rip at 1x or 16x, your drive still has to turn the disc the same number of times. Thus, with DVDIdle at work you are forcing your drive to expend 256 times the energy to read that one DVD than if you let it just lazily read at 1x!
But wait, it gets worse! At 16x the drive is taking roughly 1/8 the time to read the disc (remember it is 16x CAV, not CLV), so that 256 times as much energy must be delivered and dissipated in 1/8 the time. In other words, the power is multiplied by 8, so that now your drive must supply 1024 times the power at 16x compared to 1x!
Ouch! After all that, your drive really will need a "rest".
So, in addition to the much greater forces, the drive has to dissipate much greater power which leads to much greater heat build-up, and heat is one of the big enemies of long life. Try it for yourself. Play a disc at 1x for an hour, then remove it from the drive and feel how warm it is. Then rip the disc at 16x, remove it and feel how warm it is.
A similar agrument applies to the disc itself. It gets hotter, and the forces and stresses applied to it are 256 times as great. On the whole I would say the disc would be less affected than the drive, but still...
In summary, DVDIdle's claim to fame is bunk. It does just the opposite of what it claims to do, and will actually shorten the life of your drive. Even if its claimed benefits were true, you would have to ask yourself whether or not they would be worth the price. At $30 for a new OEM 166S, does paying $20 to theoretically extend its life make much sense?
Whether or not DVDIdle is worth buying for its other features I don't know. NoSmartz says he likes it for decrypting, but there are free apps that will do that as well.