There are many different types with varying numbers of studs. My experience in the UK was that I never even thought about needing them (mind you, I've only had 3 offs in my life and two of them were down to black ice, but that was when I was living out in the sticks).
The people who make their own with screws etc. are usually living in places like Manitoba where it gets down to -40 in the winter for 5 months of the year and there is permanent thick ice on the ground during this time. Where I live in Ontario, it only gets down to -25 in the winter, and most of the time it's more like -15, and the ice it sporadic and variable - mind you I can ride on the lake with my studded tires!
FWIW, I bought the Innova ones first and they were crap: really difficult to fit and the bead sheared off from the rest of the tyre, plus the studs rusted really quickly. The Nokian ones are the dog's bollocks. I use Hakkapeliitta W240s. Schwalbe are also really good. You have to remember to ride them in for 25 miles on ordinary uniced paved road before they bed in properly... a lot of people forget and wonder why they don't seem to 'work'