I have no idea what the symbolism behind it is. I didn't even know the words keffiyeh or shemagh, though once I googled it I recognized what I saw.
As far as impressions go, if I see one being worn around the head, I assume there are religious or practical reasons, and if I see one being worn around the neck I assume there are practical or fashionable reasons.
Other than that I don't really have positive or negative thoughts about them. I suppose it's possible that wearing one could be part of me mentally labeling someone a hipster, but there'd have to be more going on than just the one item of clothing, and even then it'd be idle speculation, nothing I'd actually base my responses/actions on. I wouldn't wear one myself, but that's due to how I feel physically having stuff by my neck, especially something with lots of little fringe bits. It would drive me nuts.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack