If the answer is Thomson...
...Should you have asked the question?
Made in the USA to exacting standards
Backed by a company that stands by their products, no questions asked
Seriously though, I have been riding X4 stems on my road and CX bikes since the DAY they were available. NEVER has an issue. No bar slippage, no steerer damage (talking to you FSA). It's an install and forget it piece.
If there's a bike in my quiver, it's wearing Thomson stems and posts...well, except the seatmast bike! :)