I received my Oneup 42 tooth cog a few weeks ago and it’s brilliant. Well made and works flawlessly. As I am not a pro athlete or internet hero I found that my ageing 41 year old carcass was getting a bit knackered in the mountains on a 36 cassette so I thought I would give it a go (1X10). People were saying that £60 for one cog is a rip off but it’s infinitely better than being dry bummed by SRAM for a whole new setup.
It took minutes to fit, is unnoticeable when shifting (in filthy mud) and the only question left is how long it lasts. I have a steel freehub so freehub wear is not a worry. Your mech looks a bit weird in 42t and chain length is a fine art. I found that I needed a chain device to take up slack in highest gear but maybe just using a longer mech would work (I use med SLX).
With a 32 tooth front chainring it feels like I have about a 26t granny when using the 42t, which is ace up here in NE Scotland with massive climbs and long days out.
All in all a really great product and will be great until Shimano make a 42 tooth cassette at a fraction of the SRAM one.