Last year, when I was doing a bunch of travelling to the chilly Pacific Northwest, I decided to buy a fairly serious insulated jacket that would mainly be worn casually rather than necessarily for hiking.
The Rab Alpine Microlight is one of Rab’s lightest down jackets, with around 125g (depending on size) or 750 fill power hydrophobic down, contained in the breathable and windproof but not hugely waterproof Pertex Quantum.
It has a slim fit, which makes it both perfect for the slightly more fashionable day-to-day wear I wanted it for, but also for layering underneath other jackets, which makes it a great addition to my hiking system. It’s the warmest jacket I can comfortably fit under my waterproof shell, and also could, at a pinch, fit under my much more roomy Electron jacket. That combo, especially with a fleece underneath, would allow me to be very well-insulated in extremely cold conditions.
The model I’ve got comes with a warm, insulated hood with an elasticated back for a secure fit. It also has elasticated wrists and drawcords at the waist, so you can pretty easily secure it against drafts of cold air. Finally, two waist pockets and a chest pocket make it practical. I particularly like the chest pocket for putting a passport or ticket in when I’m travelling, and I now notice the lack of it on my Electron jacket.
Like other down jackets, the Alpine Microlight is highly compressible, and comes with a little stuff sack. It’s particularly great when going on long runs, or run-commuting, and wanting to be able to take a warm jacket with me but taking up the absolute minimum of space.
I’ve worn it with just a jumper underneath in temperatures getting close to freezing, and for day-to-day use that’s probably below what it’s really suitable for, although for active use I suspect you could happily wear it well below freezing as long as you were staying on the move. Otherwise, for general wear, it’s best on chilly but not freezing days, maybe 8 degrees C and up, depending on your own tolerances of course.