I wanted something with a low profile I could use under an espresso machine that was good looking and accurate. The app feature wasn't important, but water resistance and portability were. I also wanted a rechargable battery vs replaceables, and something with a built in timer. Did I mention I wanted something good looking?
The Black Mirror is everything I wanted in a scale. I have had it now for just under a month and have used it nearly everyday. Today (as you can see from the pictures) is the first time in that period I will have to charge the unit. Obviously that is based off how often you will use it, but I set a 4min timer for each pour over with an average of two/day, plus I use it when roasting and all the other things a coffee scale would be used for...
It's fairly accurate. As you can see from the video, it does have a bit of inconsistency +/- 0.1 g which depending on the application could be meaningless or crucial. I argue that if getting a weight accurate to within 0.1g is crucial, then a scale measuring into the hundredths would be more important. For me, this is a non-issue.
I will also say that it is not quite as small as the Acaia or especially the Felicita, so if space is truly a premium for you, then this may be bigger than you want. I can't think of many espresso machine surfaces that this wouldn't fit beneath, but you may be in that group, so I hope the pictures help with that. I will also add that the device has a "sweet spot" for turning it on. Before I found it, I was poking around the surface trying to figure out how to turn it on. I can't stress how minor of an issue that was, but I'm just throwing it out there because it's really hard to have anything negative to say about this thing.
If you want something that has an app interface with 100% accuracy down to the 0.1g, then this isn't for you. For all other features of a good coffee scale, look no further. It is the best value I think you'll find.