XX1 on my Mtb and it's totally awesome and smooth. The all steel cassette is bulletproof and the largest cog is alloy but easily replaced. I hopped on my Mtb a few weeks ago to do a roadie group ride, chose the Mtb as a handicap. Last time I rode the Mtb was in a 12hr race and it's just sat there collecting dust. I didn't even Chilean the chain, just added some lune and wiped it down and I was amazed at how quite and smooth the drivetrain was.
Most people who say 1x11 is flawed are ether just old school haters or completely ignorant of how to tune a bike.