I was a member for a couple of years. My personal view is that it's only worth it if you live in, or near London.
The main benefit of joining is having access to the Vaughan Williams library, and the various events at Cecil Sharp House.
If you live in the north, this isn't practical and all you'll get is the annual 'academic' Folk Music Journal and a sometimes interesting, but fairly rubbishy quarterly magazine. £30 is a LOT for those (especially if you have a local library that gets them)
Of course, joining as a club or festival is a different matter entirely.