Any question answered...

I bought some years ago but only rode them once. They are for hard packed ice/snow, you will probably end up losing most of the spikes if just riding on lightly frosted tarmac.

For unpacked snow big knobblies are better.

But I used Schwalbe Marathon Winter spiked tyres for commuting in Winter when there was ice about.

The first km of my commute (from home) was generally on ungritted roads, and so there could be big swathes of ice. The Marathon Winters had spikes either side of the main ridge, so they didn't contact the ground in normal riding (with enough pressure in the tyres) but as soon as the wheel started to slip, or you were cornering, they'd dig in and stop any slipping.

Once I was onto main roads that had been gritted/cleared the studs gave a bit of whirr and were harder work than normal tyres, but I was happy to put up with that.

But the studded tyres gave a huge amount of confidence in snow/ice conditions. I remember cycling one-handed up a hill past a Police car that was just wheel spinning at every attempt to move forward. They wondered how the hell I was keeping upright.

I probably only used them about 5 days a year but they made the difference between being able to get in to work by bike or being forced onto rammed/damp/cold public transport with everyone else. I had them on a spare set of wheels so it was easy to change in the morning if I decided I needed them.

There are others (Nokian?) that have a line of spikes down the middle. They'd be really tricky to ride on tarmac, but the Marathon Winters were good as long as you had enough pressure in them.

They most definitely work on tarmac when conditions call for them, I mostly rode them in Sweden though. You'll need to bed the spikes properly first to avoid losing them.