I get cold hands. When I was young I could ski all day with gloves wet enough to wring water out. In my 6th decade, things aren't like that. Cycling with regular ski gloves doesn't cut it. Reviews of serious cold gloves and mitts that a few people complained aren't warm enough moved me to try something nobody said this about.
I went out in -6F windchill today. I exposed my bare hands to cold getting my bike stuff ready before heading out, so I gave the alti mitts a major test, i.e. to warm my cold hands, rather than merely keep them warm.
I started with the liners. This worked for the right hand, but left was still cold after a few miles, so I put the outer shell on it. This worked, and I was then able to remove it and just use the liner, and my hand stayed toasty.
Later WC went to 15. My hands got sweaty, so I switched to the windproof Gore-Tex shells over thin liner gloves.
Braking, shifting, and adjusting zippers worked fine.
Take-home lesson: the liners alone are fine for sub-0 for a person with cold-sensitive hands. Shell and liner combo should be good for another 30 degrees lower.
I looked at Burton AK Mitt, which I'll bet is also toasty, but it only has a 1 year mfr warranty. BD Guide is also 1 year. OR's is lifetime. For this kind of money, I think lifetime coverage is called for.