I use them and love them.
I put them on and fold them back like the does at 0:47 I ride for an hour like that, then when I'm getting cold I flip them over.
I love them for doing lines, untangling lines, attaching pigtails. I have a massive fear of not being able to feel a line in a situation, or being able to untwist something or grab a line. I've had lines slip through gloved hands because I couldn't grip them hard enough.
The glove/mitt will fill with water so you want to have the wrist on the outside of your suit, this keeps the flushes out of your arms.
Also, when you are riding, take your front hand off the bar and place the hole over your mouth. Some long slow out breaths will warm your hand right up. Ball it into a fist to capture the warm air, then give your arm a swing and the blood will go back into your fingers. This really extends my session. If you get your nose into the mitt, you can warm that up too
Some sessions I don't pull them down, just have them there in case it gets cold.