Danielwrote a review of Shimano GRX RD-RX812 Rear Derailleur on October 7, 2019
Familiarity: I've used it several times
Gravel has been the fastest growing segment for the past few years and Shimano has finally done something about it. GRX is a much needed drivetrain but doesn't quite go far enough with gearing for us casual riders in the mountains.
To start with, performance and looks are on par with Shimano's road groups, so really good. It's a very clean looking groupset. So if you've liked your Ultegra drivetrain but want easier gearing, going to GRX makes good sense. There's some compatibility between the two groups too which is nice.
This rear derailleur interestingly enough has only a 5mm shorter cage than an SLX which fits a 46t cog so despite Shimano's stated max cassette size as 42t I plan to try it with an 11-46t eventually just to see what happens.
I've had a 105 shifter paired with an SLX rear derailleur and 11-42t rear cassette (using Wolf Tooth's excellent Tanpan adapter) on the bike until now so I already knew what to expect for gearing but GRX really does shift crisper without that adapter and this group feels lighter weight.
So with no substantial complaints on the quality of this group my big gripe is the gearing options. Shimano has provided good options for a lot of riders on flatter terrain and for gravel racers but us bikepackers and for those big days in the mountains we need better options. Sram's AXS drivetrain mullet builds are popular for that giant 10-50t range, Shimano should have used their Micro Spline freehub to allow for a 10t cog and made the derailleur for 46t cassettes, then with more chainring options they'd have a perfect, more versatile group.