It is a good choice for those on a tight budget, or who like to use their muscles. Outstanding portability and value for the price.
Positive Product Points
For 35$ I have a grinder that can produce a wide range of of grinds and is functional in the espresso size range (you can get it to produce a 20-30 second pull, but not a consistently good shot). Small (holds ~24 grams), portable, very easy to incrementally adjust and clean. Based on a number of professional reviews it appears to be equal or superior to electric grinders < 200$.
Negative Product Points
The incremental grind adjustment has only a few settings within the espresso range. The grind size distribution is much broader than with a high quality grinder (e.g., Baratza Virtuoso Preciso).
Espresso performance: I bought the Hario for use with my Gaggia Classic. I found experienced user reviews for this purpose Jeremy's Blog finds it superior to his 200$ Kitchen Aid at the finer settings. There is/was also a video at Seattle Coffee Gear in which they tested the Mini (and the very similar Skerton) and found the pull to be "pretty good" and the consistency of the grind to be excellent.
I used it for ~ 6 weeks before buying a Baratza Virtuoso Preciso. The Hario is usable, but it does not come close to the quality of the Preciso. I would now call the Hario acceptable, and clearly a much less precise and controllable grinder. Bottom line: There is a substantial difference in the quality and consistency of my shots.
Prior to using the Preciso, I made a photo of various grinds alongside a comparison espresso grind from the nice people at Peet's: Grind comparison photo. The photo labels: Peet's in-house commercial grinder set to espresso (setting #3). The Hario Slim numbers refer to the number of clicks backward from the finest setting. Size standards: dimes and table salt. All are identical magnification.
I've also used the Hario (settings 6-8) to make a good cup with a Brikka stovetop.
Burrs: conical, ceramic. There is some play in the burrs.
Durability: the plastic exterior is comfortable to hold and feels strong enough to endure a fall.
Power drill: the handle sits atop a 5-sided axle. The handle lifts off to expose it. The exposed axle can connect to an electric hand drill. If connected directly there is some slippage and metal shavings. I found that the ratchet side of some sockets will make a tight connection. I then found an adapter that would fit the socket. It works well (~15g/min). Using a power drill first thing in the morning to turn a modified hand grinder is not for everyone.
Comparison to Hario Skerton (based on my reading of other reviews of the Skerton): This one is smaller, easier to adjust, has a spring to press down the inner burr, and has a cap to keep beans from falling out.
Arrived in 3 days. No problems.
Three Month Followup
I am still happy with it as a coffee travel companion, and think it is an excellent value, but I gave up on it as an espresso grinder. A Gaggia Classic is limited by this grinder.