Just joined recently and been folllowing a couple of threads. Recently purchased an HG-2 which utilises most of the what is going to be on the Key from my understanding. Same burrs, same Magic Tumbler, WDT, wiper etc albeit one is motorised vs hand ground with a the fly wheel etc, so for what it's worth I can give a few comments on what I understand to be overlapping areas/common parts:
Straight out of the box, you can see it's very well put together and well finished, pretty much flawless. I don't expect anything to be perfect as there is no such thing as perfection on this planet. After scouring many posts that HG-1 users have made, concerns about seasoning were laid to rest, it literally worked very well straight out of the box once dialled in. The grinds fall beautifully in a uniform circular manner into the tumbler and I have no need for WDT when after I transfer my coffee grounds into a portafilter - you just need to get used to the flow. Is it perfectly level all the time? Of course not, it depends on how well you swirl from tumbler into the portafilter basket.
I've had no rubbing issues. What I can see is that for example, when you say coarsen up the grind setting to make a bigger opening for cleaning (so the gap between the upper and lower burr increases), as the wiper which is attached to the smaller burr housing, the plastic wiper will - of course sit lower in the tapered shaped tumbler - which of course is going to rub - but you wouldn't normally use this setting.
Extractions have been very pleasant on variety of beans from med to medium light, Ethiopian to South American, I haven't tried the darkest of Full City/Italian roasts whatever you prefer to call them. For a typical shot it is so easy to dial in the grinder and get a lovely espresso within that 24-26 sec range (if you want to be SCA pedantic about it). From the few limited tests I've done on the VST refractometer, no issues hitting that so called "golden cup range" if anyone cares for that.
I've been amazed at the build quality, the ease of use and changing grind settings. For those wondering how fine it can go, when I first got it, I had it dialled way too fine nothing was coming out of of the group head - more like a Turkish level grind size by accident
- hope that helps to clarify some issues.
Almost forgot to add, retention is really negligible with or without RDT.