I haven't used them myself but have seen them in use.
I work in Milton Keynes and we had some pretty heavy snow up there last year. The carpark behind where i work was covered in about a 1.5ft of snow, and lots of cars got stuck, especailly on the compacted snow at the entrance and exit where it had turned to ice. The only cars that weren't getting stuck where 4wheel drive vehicles like audi qauttro's etc. But i did see a 3 series with snow socks on the back drive wheels driving over the snow and ice in the carpark without any issues and did not get stuck once.
Also see plenty around town on FWD cars aswell driving around as normal. So i would say they are a good idea to keep in the boot and get you out of trouble, and tonnes cheaper than fitting winter wheels and tyres.
Hope this helps