I purchased the Optiview Stowaway, and received additional Stowaways in the end-of-the-year Arktis gumball rotation.
There's a good reason the KS staff speak so highly of these. I've used the stowaway for recreational, professional, and volunteer purposes. These work great on their own, but also nest wonderfully as a base-layer.
As an advanced user note, Arktis suggests using DWR to improve water resistance. This works pretty well! But keep in mind this will remove some of the intended breathability. This may affect seasonal performance.
That said, my DWR treated stowaway came in handy during one particular adventure in the White Mountains of NH. An unexpected storm arrived, dropping literal buckets, the type of storm that removes visibility; a miserable situation. The stowaway kept me going and kept my core dry! I was the driest person at lunch.
I've compared these to an alternative item on the market, by Helikon-Tex, the "tramontane." The competitor item is about half the weight, a 100grams less, than the stowaway. Thing is, in my experiences, this proved too light. The extra weight in the Arktis achieves a far better balance between weight and performance.
These rule - there's no excuse to not have one in your kit. It'll fit anywhere!