I used to sell a lot of soax stuff, they are by no mean cheap or poor quality (as someone mentioned) but not widely used in the VAG world. Used a lot in motorsport, especially rally/tarmac rallying/ grasstrack etc...
Overall quality is good and made in Uk, but if I had to pick up on one point is that they are often a lot harsher than other springs such as eibach/h&r etc...