This accessory feels a little flimsy/cheap for a Weber product... But it does its job well enough on the Key 1 grinder. I have had really good results slowly feeding beans before, and now it's less work. Beans can get stuck with this sometimes, but I'm usually watching anyways, so it's not a big deal for me. Anyone who buys this is probably already very involved in their coffee-making workflow ;)
If you have a Weber grinder, just go for it! It's worth it just to experiment with.

This helped with my biggest irritation with the grinder - having the grinder stall at low RPM. The feeder slows the rate of beans entering the grinder so it no longer stalls at low RPM. It also helps with the messy popcoring of coffee fragments. So two big positives. However, I now need to manually push beans that get stuck on the outer ring, and then also need to lift the ring and manually push in some beans that get stuck in the feeder. So it’s a trade-off of less stalling and a tidier countertop for more baby-sitting during the grinding process.

I love the idea. Actually setting it up hasn’t been that great.

It isn't obvious that it’s installed correctly. It slows the bean rate, but the grinder can lock easily on espresso with light roasts. A few beans can linger behind in the feeder, and never reach the grinder.

Really should be included with the Weber Key, it popcorns and makes a mess at higher RPM even medium and low to some degree (taste is best at low to medium RPM anyway). I have the HG-2 now which I used it on. Which is only low RPM but still popcorns without this occasionally (not as bad as with the Key).

On the Weber Key this is more useful, since some people have issues with stalling on it at lower RPM for espresso, so this may help slow the feed rate and make it easier for the grinder without stalling, as well as prevent popcorning. Mine didn't seem to have much issues with stalling at least, but my friend hated his because supposedly it stalled often.

It also theoretically can improve grind quality by doing a more gradual / slow feeding of the coffee and a more consistent feed rate. Not sure I noticed a difference, but may increase flavour clarity (and possibly reduce body further) which is already quite high for a conical burr grinder (and quite low body already).

Kinda was a pain in the bum to install, very tight fit in the HG-2 not sure I'll be able to ever remove it if I wanted to! Seems expensive for a cheap feeling light weight couple of plastic pieces, still MUCH more affordable buying from Eight Ounce Coffee and getting their awesome customer service rather than buying from Weber which will cost you almost 3x the price! (excessive shipping plus duties) and may have questionable support!

It doesn't seem to slow down grinding (at least not by much, with both shutters opened) but I feel like it might make it easier to grind on the HG-2, even on the 'easy' mode with the 2:1 gearing on the thing, it's a bit of a workout for Light Roast grinding super fine for espresso, but doable. For pour over's its super quick and easy.
I'm happy with this mod now once it's finally in, It's relatively hard to adjust the shutters open / closed and there's two slots you can adjust, I thought it was stuck at first, but it's just tough, but not something you'll do all the time. A bit worried it might fall apart and get into the burrs, hopefully not much pressure is being applied on it while grinding. I had one of the friction plate things fall off the sides while trying to fit it in. Maybe it fits the Key better than HG-2? Or I did it wrong, or it's just how it is, I dunno.