I used a variety of gloves and mitts this year in the NW Passage. This is what I found to be useful for cold-weather sailing.

As a general rule, insulated gloves of various kinds work well in temperatures 8C/45F and above. For colder temperatures, mitts are the only things to keep hands warm.
Note that I don't pull on ropes with any of these gloves or mitts--I don't think any of them will last long if used for pulling hard on ropes with. As most of ones time is not spent pulling on ropes, it isn't much of a problem to remove gloves/mitts to pull on ropes (reef/tack/gybe/etc), then put the gloves/mitts back on to warm back up.

Sealskin Mitts with Felt Lining (top left):
Not generally found in stores, I got these in the Northwest Territories many years ago. One of the mitts is now missing the (removable) felt liner--otherwise, these would have been the warmest mitts I had. Excellent for keeping the hands warm (especially with a spare set of liners so one can be drying below while the other is in use), and very easy to get on and off. Too hot to be worn above 10C/50F.


Rabbit Fur Mitts with Polyester Insulation (middle left):
These are made in China, and sold in Canada (and probably USA) at outdoor equipment stores for about $30. They are the warmest mitts I had aboard. Even when moderately wet they were still warm (though when it rained a lot, I switched to the blue insulated rubber gloves). Too hot to be worn above 10C/50F.


Insulated Yachting Gloves (top right):
I got the West Marine insulated gloves several years ago and never used them until this year. They work well in mild temperatures, and are a bit warmer than the insulated rubber gloves when it isn't really wet. Quite comfortable to steer with. Warm down to about 8C/45F.


Blue Insulated Rubber Gloves (middle right):
I got a pair of these in Rimouski, Quebec for about $10. I only bought one pair because I wrongly figured I could buy another pair along the way at any fishing supply store. It wasn't until Alaska that I found another pair. Excellent in wet weather, and warm to about 8C/45F. Loose and stiff enough to be easy to take on and off. They take a long time to dry the insulation out, though, so best to have more than one pair.


Green Plastic Gloves with Separate Cotton Liners (bottom):
In Cartwright, Labrador, I was told all the fishermen had switched to using waterproof gloves with separate liners, that way, when the liners got wet, they just changed liners. Though I'm sure it worked for the fishermen, I didn't find this combination all that useful. This combination did work, but it takes a while to take off both an outer and inner glove, so didn't work all that well with my habit of taking off gloves/mitts before pulling on ropes. If I only took off the outer glove (which was easy), then I'd wet the cotton liners handling the wet ropes (and wet cotton has no insulation value). This system would work better if one had enough of the outer gloves aboard that one could just wear the gloves while handling ropes, let the gloves wear out and replace the gloves as they wore out (buying replacements as you go along is not practical in the NW Passage, where you can't rely on being able to buy anything, so if you don't have it with you, you may need to do without it).