There's already a lot been written about Rab's Proflex fabric so I'm going to be brief – it's fabulous! I took the Kinetic Plus jacket to Iceland for a backpacking trip in September, the first thing you note is the 'feel' of the fabric, very soft and definitely can’t be described as a Hardshell, it's also very stretchy making for a hugely comfortable garment. The main story though is breathability, with stats more akin to a softshell jacket than a true waterproof – which this is.
Unless it was a warm day in Iceland (!?!) I decided to wear the Kinetic jacket even if it wasn't raining, I wore it over a Helly Hansen Lifa baselayer and was comfortable all day, a good test of the jacket's breathability. On the first day of rain I wore it for 6 hours, of which two hours were continuous heavy rain, I could see the garment move from beading of water droplets to seemingly soak into the fabric – I was convinced that I was getting damp inside – when I finally took it off at my next campsite I was amazed to find I was totally dry.
The worst day of rain I experienced was really bad – 'blow you over' wind with driving rain for most of the day – I walked for nine hours because upon reaching my intended campsite I was told to hike on because the weather was too bad for the exposed site. I pitched my tent in these conditions and got inside, I removed my jacket which was soaked on the outside – inside I was fairly damp in places, but to be honest I wouldn't expect anything else whichever jacket I was wearing – the waterproof performance of Proflex is up there with other hardshells.
Feature-wise the jacket has all the right bits, except there's no drawcord at the hem. The hood is fine but I wore a waterproof cap under it as I do under any waterproof hood as the cap peak gives me much more protection than a hood peak. The pockets are good but not in a great position for rucksack wearers, though I understand Rab will be releasing new styles in the Proflex fabric in Spring 2019.
At 334g for an XL it's a very lightweight shell, although not in the Ultralight category it is perhaps more suitable for longer treks rather than the absolute lightest option which may not be as durable.