Always taken it out, never needed it. In these days when there are so many scams, number plate cloning and the really dodgy 'contract hire' brigade, and ever-larger lawyers fees, my fifteen years-worth of contributions to something I've never claimed on amounts to about two days fees for a good solicitor. For me, that's money well spent.
Look at it another way; I've paid out for fully comp for over 30 years, and in that time made three claims. I could have settled all the claims with the extra money I've paid over the saving for a lower level of cover, but I still insure fully comp. If you really want to save money, insure third party only. HTH