The 12 is a beautiful brush for eyeshadow application.
It applies colour over the lids nicely and sufficiently diffuse for the powder shadows I prefer for that initial laying down of base shadow. It also feels lovely over the delicate eye area, as it is made of pure and rare kolinsky (aka weasel) hair.
Since I have a typical East Asian double eyelid, I initially thought it too large, but the size is about 1/3 of my eyelid, so it is nicely sized for a base colour. Depending on the size of your eyes, the Shu Uemura 10 might be more suitable.
The price is mind-boggling, but if you're interested in a quality brush and you're sufficiently invested in eyeshadow application/makeup, it's worth it. That phrase has been used a million times to justify rampant consumerism/sell people on things they don't really need, but I think in this instance, it is trite, but true.
[EDIT] Now that I've gotten more into multi-colour eyeshadow palettes and slightly more complicated eye looks, I have to drop my rating from 5 to 4. The 10 is more versatile because of its size for my eye and as such, I reach far less for the 12 than the 10. However, if you have larger eyes, more lid space, or eyelids typified as "Caucasian", then the 12 might suit you more.
Really impressed with the quality! Soft and doesn't prick my eyelids or the outer corners of my eyes! Picks up colour really nicely, and is soft yet firm at the same time so it blends really well too.
It might be the only eyeshadow brush I would ever need, cos I normaly do very neutral eyes and dun do very fanciful colouring on my eyes :p
But becos I'm so impressed by the excellent quality of Shu's brushes (I have only 2 so far) I am tempted to go get the #10... Wanting to build up my collection of Shu's brushes, the only thing putting me off it's the huge price tag!
This is the only eyeshadow brush I need. I don't wear 5 different shadow colors and I'm not trying to create the illusion of a totally different shaped eye. I personally think 'contoured eyes' look clownish and very 1980s. I just like a nice wash of a pretty color to enhance my eyes. This brush is perfect for that. It (and actually all Shu Uemura brushes) are constantly on 'best of' lists on the Internet and in magazines, and it really lives up to the hype. This brush is an investment, though. The bristles and handle are all extremely high quality - the best you could buy. It's like a Mont Blanc pen. The No. 12 shadow brush is crazy expensive (over 100 bucks for a single brush is crazy expensive in my book), but I'll never have to replace it (my mom's been loyal to Shu brushes since they were first available - and she owns and operates a well-known beauty college, so she knows her stuff). Plus, the application and finish just look superb. I can't imagine using one of those little plastic sponge tipped applicators or some shoddy shadow brush after this. My only beef is that I would like the case to be a little more substantial. I travel frequently, so I can't exactly keep my brushes in a little jar or cup like some people. And anyone who has packed a brush can tell you that you have to be careful not to ruin the shape or slant of the brush after you've put it in your bag. But other than that, it's perfect.
This is a very nice synthetic brush that works well to cover relatively large areas with eyeshadow. It's a big brush, 3/8" wide by 5/8" long with a rounded head.