This jacket is perfect for cyclists who get caught in rain on their daily rides, perfect, packable size (super small, fits nicely in the bottom of a jersey pocket). Very light and breathable, obviously for use in the warmer months. The fit is described as "pro-fit (slim)" on backcountry.com but is definitely lose and baggy. I'm 5'5" 126 lbs and the small is very baggy on me... which works for me, I'd like my rain jacket to cover the things I have in my jersey pockets (keeping them dry) and still have the ability to zip it up in the front.
This stuff is legit... it doesn't have any of the garbage that most drink mixes have but it also has good flavor (unlike Acclimate or other products based on similar science). This stuff got me through my first 24 hour mountain bike race last weekend... I almost went through an entire bag and I'm still not sick of it!