Been using Schwalbe Ice Spikers for the past two winters and they’re ace on both hard-pack snow and particularly sheet ice. Nothing special on softer snow, but then nothing is. They were superb for riding back lanes in the Peak on days when the trails were just drifted to hell. A little heavy and draggy on tarmac and make a fierce noise, but nothing dreadful and very, very durable even with a fair bit of riding.
They do have limitations, as someone said above, they’re not completely idiot proot, more like riding on a slightly loose, slippy surface than having ‘normal grip’, but you can happily ride on sheet ice. Last year, I noticed that grip reduced as temperatures went below -10?C or so as the sun went down, measured by the level of uphill angle needed before the rear span out on me…
Over Jacob’s on sheet ice and up Chapel Gate and back over the Roych on Christmas Eve was ‘interesting’, at one point I found myself lying in the middle of a ten-foot wide stream of water ice and having to basically crawl off it dragging my bike behind me…
Worth it? Over the last couple of years, definitely; let me ride trails I would have had trouble walking along. If this winter’s anything like those and you want to keep riding even when everything’s frozen hard, they make a lot of sense.