Rukka Kastor 3 Armoured Vest

Quality is brilliant, sizing is bob on. I'm 6'1" with 47" chest so went for the XL and it's a good fit. Reason for purchase is that I've three jackets which are different brands and require different back protectors. I thought this would be a better investment with the added chest protection which my jackets don't cater for. Admittedly in my summer jacket the chest pieces make me look a tad ridiculous as I'm a pretty big guy already so I have taken them out for shorter rides (Very easy to remove/replace with zip pockets). That being said, although the chest pieces offer less protection than the single piece protectors they are far more comfortable and for me that helps me concentrate on my riding which is more important. The Cordura fabric keeps the flexible D30 protectors firmly in place and the breathable material makes the vest comfortable to wear on longer rides. I'm really pleased with the purchase and consider it a solid investment.

I think this is a great item, so as there's no reviews so far, here's my take:
I didn't know this existed until I purchased a Rukka Forsair Pro jacket and started searching around to find chest/back armour to go with it (jacket only comes with elbow & shoulder). Initially I was looking at just getting the armour inserts, but stumbled across this and by the time I'd priced those up, the cost was already climbing towards the price of this.
This is a great solution to having to swap back inserts between different riding jackets (summer/winter) or having to buy a new one each time I get a different brand jacket (which takes a different shape back protector). Also the added bonus that it covers the chest, which few jackets seem to cater for. Hoping it pays for itself in the long run if I don't need to buy any more inserts!
The vest itself is well made, the high quality as you'd expect from Rukka and the Cordura fabric does feel hard wearing, so no concerns there. I'm 6ft, ~40" chest, so an EU size 50 in Rukka (and Alpinestars) fits me nicely. I picked up a medium Kastor 3 and the fit is perfect; not quite snug, so not restrictive, but well enough that nothing will move around. Looks like the vest will have a bit of give around the stomach in case you don't have an ironing board, but anyone with a hefty beer gut may struggle…
It sits beneath the Forsair well and doesn't really add extra bulk. Being D3O the pads mould to your body shape somewhat which also helps with the close fit. Overall very comfortable to wear beneath a baselayer, but also enough room for me to wear it over a thin/medium fleece as well. No problem keeping this on even after I've taken my jacket off.
Haven't had opportunity to try in blistering hot weather yet, only mid-teens which was fine. Even with it having Outlast I guess it's another layer so you may feel it in the 20's. But, having said that, with the Forsair (or any other high vent jacket), I really doubt it would be an issue.
I did try the Kastor 2 on as well, I suspect it gives (slightly?) better protection of the chest as it's a single piece (so no gap for a zip in the middle), but the downside is I think that caused it to not fit quite as well, I preferred the Kastor 3. I'd also tried a small on (according to the size guide it should just have fit), but I'm definitely a medium.
Also tried this under my Nivala and this fits really well, will probably use instead of the stock Nivala inserts as it's a better fit and more comfortable.
In summary, great item, very comfortable, not (too) far off the price of having to buy individual inserts and is transferable between jackets, also gives added bonus of chest protection if your jacket doesn't support it.