I personally do not feel its worth it. Hear me out.
Firstly lets look at the premium. You can get GAP for under £100. Yet they'll pay out many thousands when you claim. Bargain! But of course do the sums - for it to make business sense they must have a very low premium:claim ratio, therefore they take in more money in premiums than they pay out in claims. Given the low premium and high claim value this suggests very few people ever need to claim for GAP insurance.
So, first point - you are very unlikely, statistically, to claim under GAP insurance.
Lets move on to what it protects you against. Return to invoice isnt actually protecting you - its putting you in a better position than you were before the accident. This is nice to have but it also means that if you didnt have it it isn't the end of the world even if the worst happens.
Buy a car for £12k, a 1 year old Mondeo. Crash it 2 years later and get paid the value of a 3 year old Mondeo, say, £7000. You now have £7000 to buy a 3 year old Mondeo with and the net result is that before the crash you had a 3 year old Mondeo, after the crash you have... a 3 year old Mondeo. So you've not really lost out in anything other than an incidential way.
IMHO GAP is only worth considering when not having it would cause you financial hardship - ie, if you are the sort of person stupid enough to buy a heavily depreciating mass market eurobox on finance and find yourself in negative equity. Otherwise GAP is simply gambling - you are gambling that you'll write your car off and receive a full refund on the price you paid for it 2 years ago..