I bought one of these last year and after having used it quite a bit I can say that it is my favorite. I also have a lightweight Zan Headgear helmet liner, a REI beanie that someone gave me that is similar to their Thermo Fitted Cap, an Eddie Bauer First Ascent beanie, and a Kuhl skull cap. I wear this under my 3/4 motorcycle helmet while riding and sometimes wear it while outside in cool windy weather. My head measures 22 3/4" and I usually wear a size 7 1/4 hat. My head size falls between the Medium and Large size on the Outdoor Research size chart. The Pro Factor Cap I have is the Large/X-Large size and it fits slightly loose but not so loose that I want to try the smaller size. What I like about this cap is it completely covers my ears and forehead which definitely helps keep one warm. The seams are completely flat and I can't feel them even when wearing a helmet over the cap for several hours. I haven't worn this while doing any strenuous outdoor activity but I suspect it would be too warm for that, at least for me.