I joined the FSB only a week ago, I managed to recoup the costs within a couple of days by contacting my card processor and by telling them I had joined, this saved me all but £5 of the first years membership.
The main reason for joining though was for the VAT and TAX investigation insurance, our accountant was quoting us £300 plus per year but this comes free with the membership this covers you in case of an investigation, as I understand it they will pay your accountants fees which can rack up as the investigations can last for a year or two in some cases. The free legal advice service is also something to note as well as possible better insurance rates.
There is also the free banking service but as its with the Co-op I am a little nervous about moving at this moment with all that's happening.