I have come across this product while searching the net, and I wonder if it is worth to buy it. Is it a necessary to add if I want to use my popup flash?
There are set of Popup flash difuser that comes with different color sets. White, Blue and Yellow. If you want to avoid using your heavy external flash, it can be very useful. If you are convinced that you need those popup flash diffuser, why not try DIY popup flash difusser, then see it for your self if you really need it.
I think not.
The problems with the popup flash are: 1. it's too small - this makes the light source very hard, 2. it's too close to the lens - light coming from the camera's direction tends to "flatten" the image and 3. it's not very powerful.
The diffuser you linked to is still too small, still too close to the lens and will only reduce the light output of the flash -- in other words, it's not going to make much difference in the quality of the light (you need a bigger diffuser for that), it wouldn't change the direction of the light at all and only make the power issue worse.
Diffusers like these are generally trying to spread the light around allowing it to fill the room and shadows, so in a small room they may work. That is in a small room your flash will have enough power to bounce light and fill the room, filling in the directional shadows that give a popup flash the ugly P&S on axis look.
That said before you go and spend any money, in a small room with a white ceiling, grab a 3x5 note card and try putting it in front of your flash at a 45° so the light goes up and bounces down. Also try angling the card to either side -- this is mimicking a bounce flash.
Small on axis light sources are generally not pleasing as they create strong light with no interesting shadows on your subject and ugly hard shadows behind them. Also when trying to make the most of a small flash, don't be afraid to up the ISO a bit, it will help bring up the ambient and increase the effective power of your flash.