Good design idea, poor QC, your money might be better spent on a Zass or entry level electric.
Positive Product Points
This grinder is indeed slim and portable, construction is very simple, it's easy to figure out (good because the instructions are in Japanese). Grind might be acceptable if you get a product without flaws. It looks better than box grinders in my opinion.
Negative Product Points
Shaft was bent out of the box, resulting in a very uneven grind at all settings. The shaft is custom, with a key for the lower burr, so it cannot be remade very easily. Somewhat loose tolerances at lower burr/keyway, keyway/key, and upper burr and its socket. So, even if you get a perfect shaft, there are still design flaws.
I was very excited for the Hario Slim after reading many positive reviews. I have owned one other manual mill, a "Stoha Design" wooden knee mill, found at TJ Maxx for $7.50. In short, I wish I hadn't bought the Hario because my cheapo mill, and even my vintage Braun KMM20, is more consistent. My sample had a bent shaft out of the box, and for a $40 grinder, it's hard to say it's worth fixing. I'm now looking for a Baratza.
Hario mills take manual mills in a good direction, I think: they're lightweight and attractive, and they have decent ceramic burrs. Construction is very simple, so cleaning is easy. However, with the bent shaft possibility (which, after searching online, seems all too common) this grinder is a disappointment. I really wanted to like this because the size makes it a perfect grinder for travel.
Supposing you get a sample with a perfect shaft, there are still loose tolerances at three locations on the burrs. Searching reveals a few methods for tightening up tolerances (cement, tape, etc.) but I would not buy a product like this, intending to modify it.
One other nitpick I have with the grinder is that, although it fits comfortably in your hand, grinding while holding can get tiring. Knee holding works ok, but not as well as a longer box grinder.
I bought this at my favorite local third wave shop, Comet Coffee. They've always got them in stock, and for a reasonable price considering their drinks, location, and overall hype. $40. The issues I have with the Hario do not reflect my general experience with Comet: great service, convenient iPad/email payment setup, and other merch offerings!