I would not be without it on a pricey car, NEVER buy fro the dealer as above their commission put the price up massively.
I bought mine for my R from GapInsurance.co.uk for £128.38p for 3 years £20k cover with a deferred start for a year as my ins co gives full car replacement on new cars for the first year (this was through a Forum sponsor on the Briskoda Forum so includes a 5% discount). So I effectively have 4 years cover for the full cost of the car. No dealer will/can offer to match that.