First, the balaclava discussion is always predicated on saying 'it's mostly the helmet' -- I've got a new-ish Shoei and a Schuberth, and my C2 is the every day helmet. And it seals up REALLY well.
So, when temps hit 50F, the balaclava comes out. I've been through a bunch of the coolmax ones, the nameless ones at the cycle store, etc. I'll tell you that the best I've used, ever, is that Pharoah Deluxe from Schampa. Keeps the noggin nice and toasty, and it stops the wind. Period. It's also nice and thin for your head, so helmet fit doesn't change much. My C2 is alot better at changing shape inside than the Shoei is.
However... this thing is not waterproof. In a driving, sustained rain, it will start to get wet. After about 2 hours, it's soaked up enough water to start getting my chin pad area wet as it soaks water in.
In the sub-freezing temps, a heated jacket liner with a heated turtleneck plus the Pharoah, and you're set.Totally comfy, provided your helmet seals up well.
The thing I don't like about any other balaclava is that they don't have adequate neck protection, if any at all. If your head is cold, your neck is probably colder!
Oh, and the split mouth is nice if you have a comm mic.