I tested the Kahtoola MicroSpikes on my ski holiday last year where I did quite a bit of trail running after the lifts had shut and even some climbing on the piste. I found the microspikes to really increase my grip and confidence on both hard and soft snow. Like any crampon you feel more secure going up than down but if you made an effort to keep your foot flat on the snow when descending you got the best grip. As everything under the foot is metal they seem like they will last forever.
They really do take 20 seconds to fit (see the video on the product page) so if your route is mixed with snow and tarmac you can easily whip them off and run with them in your hands or stuff them in your bag. I showed them to a couple of guide friends who thought they would use them for short glacier crossings in summer when accessing rock routes rather than use proper crampons.
A great product that will do you from an icy trip to the shops to a full run in icy mountains.