I'm using pi water and I think it's quite good. My friend is using diamond, one of the difference I can tell is the water flow. Diamond is very very very slow, but pi water is fast. One of the reasons that I bought pi because it's made in Japan and diamond is assemble in South east asia as mentioned by the agent that I called before.
Quite good means I satisfy with the filter. Don't always bear in mind that water can cure any kind of disease. Water is just water, it is the thing that we need everyday, don't mislead by any advertisement that it can cure any disease. I bought pi water beacuse I would like to buy good quality water filter and drink directly from the tap. Pi water is even more expensive than diamond. If you use a water filter and still want to boil before drinking, I think Doulton is another option and the price is a lot cheaper.
Below is the link of pi water, you can take a look. If you still need any further info, you can pm me.