Man-made fibres are all well and good in this damp climate of ours, but if you want to stay really warm, then down (filled) is the way to go.
The PHD Minimus Vest came to the 'gorms with me back in May. I took it as backup insulation but I ended up sleeping in it and was warm all night.
When I went back a week later with The Boy, I left it behind (I thought it was overkill, the weather forecast being so good) and was the wrong side of comfortable in my Rab Quantum bag. I learned a lesson that night.
It also ties in with the whole lightweight ethos, and I'm paraphrasing here; stuff you carry should do more than one thing. Something like that anyway. And within reason.
Anyhoo, it's so light and packs up so small that it'd be daft not to carry it. So, from now on, I'm gonnae.