It works as a waterproof, but the fit is the problem.
I'd say the fact it is only really sold in UK accurately matches the conditions it suits which is cool and wet, if you're from Pacific NW then its comparable and so good to consider.
As to how it works, its really just a capillary force from the weave of the inner material which tends to push water from the inside to the outside. To keep that working it needs to be clean which is what the Nikwax Techwash does. The capillary force can be defeated by pressure such as from tightness, which is why its baggy, from a heavy backpack with narrow straps but if you a true BPL that's not much of an issue and by wind. The wind is defeated by the outer layer which is a windproof layer. It can "wet out" which lowers its breathability so it is treated by Nikwax TXDirect to encourage run-off.
Overall, because its two layers it is warmer, the inner layer has no / negligible windproofing so its extremely breathable, the outer layer is very breathable. The two combined is more breathable by an order of magnitude than any shell, so its good for wear-all-day-with-chance-of-rain or rain-all-day situations, if its cold enough just wear it and forget. As the waterproofness is based of fabric construction and cleanliness, it will not lose its waterproofness with slight damage or age, so they outlast other waterproofs by decades. Plenty of folks have perfectly ok working gear from 20+ years back.
The biggest drawback is its 2-layer insulation, it needs to be cold enough in balance with your output and high-energy people will find it too warm til below freezing which is obviously then not encounter water, so its more of a bimbling or just-above-freezing type context which is a very common temperature in UK.
I have Cascada for walking low-speed winter coat (e.g. into town) I got used, I got Quito for more biking and faster hiking, and a Summit hoodie + Fuera Ascent for more hiking from a car type as its just more flexible. I tend to use the Quito the most.
The traditional biggest complaint, after too-warm, is its bagginess, but the fabric needs bagginess to maximise waterproofness and you can make use of it like baggy sleeves to roll them up easier. Next the sleeves often too short, a little dumb if you're in mountains and raising hands above head, but its vast breathability and insulation means it will self-forgiving rain getting in the sleeve, you'll just evaporate and breathe it out.
You'll not get a discount at their London store and I'm not sure if they'll sell to you minus the 20% UK Salestax, more typically you get home and get the garments posted to you with a discount from somewhere else. Or you can just pay full price and walk out with it.
The pants are great, the Cascada pants, again just wear when its cold and regardless of rain you're dry and warm.
The overlayer Torres are performance-wise normal stuff but its good value for the build quality, I wear the Torres gilet a lot to boost anything I'm wearing.
Avoid their baselayers, worse of any make I've tried. Cambia is horrid.
Their fleecy reversibles are comfortable next-to-skin type if you like the look of them they're ok. I like the Explorer, I got a broken shoulder recent and an older damaged shoulder and I'm using the chest pocket to act as a sling.
Most people who buy Paramo find the comfort and convenience good and buy more and more, it has a name "Paramoholics" and its fans are widely known to be evangelical fans and so the P***** use to not provoke it being mentioned.
Be absolutely sure on fit before you buy! Notorious for sleeve length and torso length too-short