I hear great things about Buffalo mitts and they're on my Christmas list so if I'm lucky maybe I'll get some.
I bought some last year, mainly because they are very lightweight (mine are 75g), and available in olive green which I like. They're said to remain warm if wet but I emailed the maker to ask if they can be proofed. This was their reply:
"If your garment is new then it will be fine to use the Nikwax TX Direct Spray on without first washing. Please take care not to get the Nikwax TX on the pile lining as this should not be proofed."
I treated them but so far I have avoided wearing them in rain. Do be careful with the sizing if you buy them online. I seem to remember there was a size guide based on the circumference of your hand, or something like that, which indicated Medium for me, but I've found them to be a fraction short for me and I think Large would have been better.