If you are owed the money – the creditor – then:
Generally you should try and avoid signing these. However, they are not necessarily binding. If the party making the payent has concealed matters from you, or if they are merely repaying you what they were legally obliged to pay to you anyway, then it may well be that you can ignore the ff agreement and go back for more.
If you sign an ff agreement but you think that you have been ripped off then don't give up.
However, do be aware that if you have signed an F&F agrement, then going back for more will probably be quite a tough battle - but not unwinnable.
If you allegedly owe the money – the debtor – then:
If you are being offered a full and final settlement by a corporate creditors such as a debt collection agency, don't imagine that this means that they will remove any mark from your credit file. They won't. They will still stay there for the entire 6 years.
Before you consider any full and final settlement agreement, you should investigate the enforceability of the alleged debt in the first place
Best not to sign these kinds of agreement in the first place.