Coil overs are a spring and strut package in a single unit. They offer a considerable amount more adjustment than a conventional spring and damper set up.
In terms of the effect it has, its nothing short of incredible.
I have a buddy running a Skoda Octavia (essentially the same chassis set up as the Mk4) and it is without doubt the best handling front wheel drive car I have ever driven.
Do it, you won't regret it!
