The first neck warmer I owned was one of those fleece tube things from a supermarket stand by the tills, bought in a panic before a cold walk to work. It pilled within a fortnight and sat on my chest like a flat scarf had given up halfway through becoming a scarf. What it taught me is that a neck warmer only works if it keeps its shape, because the second it goes limp it stops doing the one job it has, which is sealing that gap at your throat where all the warmth escapes.
A good one starts with weight and density rather than softness, which sounds backwards but isn’t. You want a snug double layer of a proper knit, merino, a wool and cashmere mix, or a thick ribbed acrylic if that’s the budget, not a single thin layer that looks cosy on a hanger and does nothing once the wind gets involved. Check the seam where it’s joined into a loop, it should lie flat against your neck, not sit as a hard ridge that rubs under your chin by lunchtime. If you’re only buying one, go for a snood style long enough to double round twice, in a plain dark colour, because that’s the one that works with a coat you already own rather than one you have to build an outfit around.
In terms of wearing it, think about what’s happening at your collar. A neck warmer sits best over a crew neck or a collared coat, tucked down into the top of a zip so it isn’t fighting a hood or a big collar for space. It’s lovely with a wool coat and gloves, properly useful over a plain jumper on a commute, and genuinely pointless under anything with a cowl neck or a scarf already built in, you end up with far too much fabric round your chin and no visibility.
Where it falls down is anywhere smart. It doesn’t read as polished with tailoring, and under a fitted coat with a structured collar it tends to look like you’ve forgotten your scarf rather than chosen this instead. It’s also no good for anyone who runs warm indoors and forgets to take it off, because unlike a scarf you can’t loosen it halfway, it’s either on or it’s round your bag. Buy it for the genuinely cold days, not as a style decision, and it’ll earn its keep for years rather than one winter.