I had about given up on Shimano but with the release of their 12-speed SLX and XT groupsets, they once again got my attention. Over the last few years my bikes have always had Shimano XT drivetrains, I've gone from a 1x10 to 1x11 and now 1x12.
They nailed it with the 1x12. Yes, it took them quite a few years longer than SRAM but the wait was worth it. I am running the 51T cassette with a 32T chainring out here in Utah and that gives me all the range that I need. Right away I was climbing more efficiently and quicker than on my old 1x11 setup.
So you might be asking, Why go with Shimano over the competition?
Well, Shimano's Hyperglide+ design is the main thing that sets them apart. With this design you can now shift whenever you want. No need to unweigh the pedals slightly during a shift to prevent the chain and cassette from making terrible noises. The chain will move up or down the cassette without any fuss. It still seems strange to not ease up on the pedals before shifting but you can and the drivetrain is happy to get you into the gear you want.
So there's that, and with Shimano you still have the ability to drop down two gears with one push. I love that feature and use it often.
I'd say if you do not care about having the absolute lightest drivetrain setup then you will be very happy with Shimano XT for the money. Shimano XT has always been a great bang for the buck groupset and that remains true with the 8100 series.
Pros: Shimano Shifters (Rapidfire Plus and I like shifting with my index finger), buttery smooth shifting up and down, quiet, easy to setup, the performance you get for the price.. There's even a mark on the inside of the pulley wheel cage to simplify setting the correct B-tension.
Cons: A little heavier than SRAM GX. A con that I will happily live with for getting a better shifting drivetrain for less money. You need a MicroSpline freehub for the cassette. It was an easy swap for me with an I9 freehub. In the future MicroSpline will be much more common on new wheelsets so this will be less of a problem.