So I've been using it for a month now... Here are my thoughts
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The Galaxy Book 12 is an admirable effort by Samsung and potentially wonderful for certain markets.
The 12" screen is the star! A wonderfully crisp, dense AMOLED which looks simply incredible. Media consumption is a pure joy on it, and the automatic screen brightness is solid. The 128GB SSD is fast to start up, and I didn't experience too much lag with 4GB RAM (however, if possible futureproof with the 256/8GB variant).
The backlit keyboard cover: a solid effort that just needed to be, well, a bit more solid. By no means is it flimsy, but when resting on one's lap it could have been sturdier. The key travel is excellent, although certain keys (e.g. Backspace) are awkwardly placed. The track pad needs a bit of customisation but I got used to it quickly. The 4 positions are adequate.
Battery life has been perfectly good (close to what Samsung claim), but the fan kicks in with alarming regularity. It's definitely noticeable.
The S Pen proved somewhat hit and miss - out-the-box palm rejection isn't the best. Handwriting responsiveness perfectly solid for annotating with (or getting signatures from clients) but perhaps not taking pages of hand-written notes.
Windows 10 Tablet Mode isn't intuitive enough: it needs to be far more simple, effective and customisable. The traditional desktop was perfectly fine, but this screen size comes down to portability versus utility.
The speakers are only decent, but having 2 USB-C ports and Micro-SD is useful. The keyboard cover and S Pen are included in the (rather high) price but perhaps Samsung will get it right next time round.
Regardless of the hardware, this particularly product category doesn't suit my needs, but that doesn't mean it wouldn't suit someone else's. It needs some refinement but this is still a worthy effort from Samsung, albeit at a high price.
Score: 3/5