I ALWAYS take it out, not matter how much you value your car it via insurance they will not pay out over the odds, so if your car gets written off in the first 2-3 years then they will probably pay out less then what you owe on finance, this leaving you in a sticky situation. This is where GAP comes in as they pay the additional GAP of money missing so your not out of pocket.
My GAP always works out as 1 extra monthly payment so say your paying £200 a month over 48 months, it will be 3200 a month over 49 months.
Nothing major and something I will always advise people to take!