First, I have not and do not take credit for 'screwshoes'. Like everything else nowadays, I found it on the internet (so it must be true!). In fact, a quick search for 'screwshoes' and you'll find lots of blogs with very detailed instructions on how to do it, although it's not overly complicated.
I used #8 1/2" sheet metal screws for the entire shoe. Sheet metal screws are nice because they have a raised lip on the screw head which provides the bite on ice. If I were to do it again (and I might), I'd probably use 1/2" screws in the back half (heel) and 3/8" screws in the front half (toe). The sole is much thinner up front and you can 'feel' the screws (although they don't actually stick through the shoe into my foot). I did remove one screw from each shoe prior to running (beneath my big toe) because it felt like it might come through. In the end I had 13 screws in each shoe. I don't know what the magic number of screws should be but I'd guess somewhere between 10 and 20. Just be sure to place them in the lugs or thicker parts of the sole.
Since it snowed Wednesday, most secondary roads in NH are a mix of snow, slush, sand and salt with a little ice thrown in for good measure. When the road conditions deteriorate the value of these shoes becomes very obvious. You get a confident grip on pretty much all surfaces, especially up and down hills. So much so that I ran just as fast in crappy conditions today as I normally do on a 'normal' training day. Pretty much all that slipping and sliding (along with the shortened, cautious stride) that is typically a part of winter running can be mostly eliminated. They are a little noisy on bare pavement but not really annoying, but I did find myself jumping off to the side of the road a few times and running in the snow shoulder (I mean really, it doesn't take much to get me annoyed so better safe than sorry).
My recommendation: 2 thumbs up! It was worth the 2 bucks and 5 minutes of time it took and if you don't like them just remove the screws. A word of caution: be careful in the house.
