It all depends on how you define 'work'. A penis pump is definitely effective at filling your penis with as much blood as possible, thereby creating your largest erection possible, but whether or not it produces any kind of permanent change is still questionable.

A flaccid penis grows by filling its internal cavities (the corpora cavernosa and corpora spongiosum) with blood, while using the ischiocavernosus and bulbospongiosus muscles to prevent that blood from escaping. These two simultaneous actions engorge the penis and help keep it that way. 

A penis pump works on much the same principle. The basic idea is to insert your flaccid or soft penis into a tube, seal it off so that no air can get in or out, then pump out as much of the air as possible. Something has to fill the void left behind, which in this case is your penis.  The more air you pump out, the more your penis will 'grow' or expand to fill the space. Theoretically, if you could remove ALL the air in the sealed tube while your penis is inside, your penis would have to expand to fill the entire area of the tube. This of course is not typically possible, and if you really were to force out every last ounce of air, you'd likely cause damage. That said, the general principle behind a penis pump is sound and works quite well, at least in the short-term. Achieving any long term or permanent size-increases with a penis pump is not nearly as evident or absolute, however. Some research suggests that consistent use of a penis pump may help expand the corpora cavernosa and spongiosum, and may slightly stretch the suspensory ligaments, resulting in overall size and length gains, but the jury's still out on this.

One thing that is indisputable: The Clone-A-Willy Kit works exceptionally well in conjunction with a penis pump. It will allow you to achieve and help maintain your largest possible erection before molding. So if you want to capture yourself at you biggest, you just might want to try a pump.