The Trek balaclava is the most comfortable balaclava I have yet to wear. Its lightweight fabric is thin enough to fit cleanly between your head and helmet while still being warm enough for cooler weather conditions. Its brushed inner lining feels neutral against your skin so that you don't become conscious of it being there. Also, the brushed spandex material wicks quite well, keeping moisture from building up.
The flaw in most balaclavas is that they are either too tight or too loose. If they are too tight that can cause a rider to get claustrophobic. If they are too loose wind gets inside and the rider can experience a lot of heat loss and a tremendous amount of wind noise. The fabric that comprises Trek balaclava has enough give to mold to the shape of just about any rider's head. It clings to your skin without being strangling. Its cuts through the wind, blocking drafts without producing too much wind noise.
This piece of headgear does its job while fading from your perception.
My only concern is that its really only effective down to about 38 degrees. Any colder than that and you'll have to double up on head gear. With this balaclava I have added in a skullcap and that was enough to go to colder temps. So, I just wish that Trek would make this balaclava a bit heavier to make it work for more extreme cold.