IMO it's well worth joining. I've been a member for several years now. The FSB was a big ally when there were drought issues.
One drawback is the payment. It is taken once per year from the nominated account. Although it isd done on the same date, there is no prior warning given. Funds were tight and it put me overdrawn a couple of weeks ago. Fortunately, I spotted it straight away and transferred some money from my personal account to plug the hole. Because Abbey now have instant clearance between accounts, I got away without bank charges.
So no warning of pending payment. No receipt issued. I have to photocopy my bank statement with the debit on it to act as a receipt. For such a professional organisation, I find this one small drawback quite amazing - especially as they are for helping small businesses.