The Bottom Line: If you want the warmth of mitts but hate the clumsy feel, these are the solution. Plus, they are ultra durable and comfortable.
These have been with me for several seasons of ice climbing belaying and ski touring. They are my go-to downhill/back-up glove. They have endured temperatures as low as -28 Celsius, and they have kept me plenty warm. Without a shadow of the doubt they have the best leather gloves on the market. Durability, versatility and warmth are the name of the game.
I went with the three finger because I wanted the warmth of a mit, but I was using these for belaying ice climbing, and having a digit free to open and remove carabiners seemed like a good idea. In practice this works really well. Your index finger normally does most of the work, so I hardly ever notice I'm wearing a part mit. It also helps that the gloves quickly take the shape of your hand. It's really cool the way the glove begins to mold to the shape of your hand after a couple months of regular use. It makes them exceptionally dexterous for such a warm glove.
The cuffs go back a long ways, which is great for deep, dry powder. There are cinches on the wrist and on the end of the cuff for double protection. The wrist guards are frustrating. They are clunky, and while easily removable, simple cord ones that you could open super wide would have been better.
The liners are removable, so as the insulation packs out, you can swap in newer, warmer liners without loosing the broken-in, custom-fit leather. This also makes drying them out way faster. On wet ski days at remote huts or coastal camping trips, this is an awesome feature since you can put them by the fire or stuff them in your warm sleeping bag.
Overall I've been really happy with the warmth of these. I have had a couple of days where my hands have been cold, though this is generally when I don't wear enough layers. Belaying with many layers at -28 Celsius is the most extreme I've experienced, and these were nice and toasty.
Weather resistance is another huge plus. Because the liner is on the exterior, these don't let the insulation wet out in gross conditions. The leather is the only part that can leak with enough pressure, though it's generally pretty good. Even building snow caves, carving with my hand, it takes at least an hour of regularly stuffing my hand into snow before they wet out. The bright side is they also dry really quickly; if I got them wet and kept moving, they'd dry out on my hand.
The Heli Ski is superb glove, and I am thrilled with them. I would highly recommend them.