Like previous reply, I partially agree. As it really depends on your municipal water quality.

It does help indirectly for your wife's allergy situation, though actual impacts can vary.

Let me give you a few example.

5 micron filters (regardless of any brands of water filter as long as it works as it should) is IMPORTANT in states like Sabah / Kelantan. Reason being is that municipal water quality is relatively low compared to other states with more developed water infrastructure. Sabah for example, experiences frequent episodes whereby water cuts occur during heavy rain due to poor water quality at the intake valves hence treatment plants operate in lower capacity. After restoration, most of the time, the initial flow of water is significantly more murky compared to general water quality.

So, even though you may have improved drinking water quality, but what about the water you use to wash your dishes? Bearing in mind that most drinking water filters are not necessarily nearby sink / made for general use. You would still possibly consume any impurities or allergens that could potentially cause allergies.

Now, let's move slightly further out. What about your laundries? You may not be consuming any laundries, but the fabrics is in direct contact with your skin, some up to more than 8 hours at a time. Let alone those that in contact with more sensitive parts of your body. Impurities that is in your fabric contents, may be caught in your fabrics, bringing it into contact with your skin. Indirect but possible causes of allergies.

Therefore POE water filters can be important, but it depends on how good quality is your municipal water source is. If you live in a mid-high end condominiums, most of which does have water filters installed prior to water entering tanks within the premise, then most likely you won't notice significant improvements. But, if you are situated in places that experiences frequent water disruption and poor municipal water quality, chances are you will be better off with a proper one installed.

Yes of course, as long as the product fits your bill after you do your research. I do not have extensive knowledge about many other brands of products so maybe other sifus here can advice you better when it comes to other brands / specific products.

For me I am somewhat a water enthusiast but not loyal to any particular brands. I like to do research about water quality / water sources / water management / wastewater management / municipal water distribution and design etc. Just so happens that 3M is the most straightforward and fits my criteria well that I don't have to subscribe and I can consider their qualities are generally reliable and consistent compared to other brands.

Whereas for Water quality in Selangor I would consider it as acceptable in Malaysian standards. Similarly applicable for Klang valley water sources, along with Perak, Johor, Negeri Sembilan, Melaka, Penang and parts of Pahang and Sarawak. Not sure about Perlis & Kedah, but for Sabah, Kelantan and Terengganu, its below standards for me.