I've used it on & off for 18 months now on various bikes.
All started with no chain device, all have a top guide as a minimum now due to dropping chains.
Cassette wear seems good, as does chain wear. Shifting as mentioned isn't as good as Shimano 10spd & the mech needs a lot more love than XTR to keep it running sweet. All of them have developed the common annoying 'clunk' when sat on the bike (it's something to do with chain growth & the clutch system). Few seized jockey wheels & one mech write off so far.
Ratios work well for me with a 34 or 36T front ring. The GXP crankset is a real pain though, every single bike i've ridden the BB creaks. The cranks arn't a patch on Shimano.
The idea is good still & the ratios work for me. I just wish it was made by Shimano, rather than SRAM. It might work properly in average UK conditions then.