I love my Rab Quantum windshirt. The cuffs were a bit tight for pulling up my arms, but it kept me warm and reasonably dry in a lot of different situations. I do find that my ears get super cold, however. In fact hoods in general add loads of warmth to a clothing sytem by protecting the face. A hooded windshirt is the answer, but at a weight/cost that didn't seem worth it. The Tachyon compares reasonably well in terms of price, but knocks 20 grams off the weight of my Quantum.
It's incredibly thin. Pertex Quantum- a fabric I found incredibly lightweight when I first felt it- feels way more robust. Light passes through it easily and it compresses amazingly.
The zip isn't as long as the Rab, which was a half zip, but I generally didn't need that much venting.
The hood fits really well, although it does have a bit of an odd ball shape when cinched up. There is a velcro tab to reduce volume at the back.
There is a little peak formed by a cord in the front. It's too floppy to stand up to wind but it's a nice touch and avoids the train spotter cagoule look.
The cuffs are elasticated. They feel a little rougher than the lycra binding of the Rab, especially at the seam. I can fix that with a little home mod though.The bottom hem has an elasticated drawcord.
It has the well designed and super lightweight feel of my Montbell down inner. It does look quite shiny and not a little crinkled. The words trash bag have been used, but I like that in a funny way. It weighs 61g in size small in it's stuff sack. The sack weighs 3g. Very nice indeed.