I have been a member a couple of times. Once early days for about 5 years, and again a couple of years ago, but only for the one single membership year, after which I resigned my membership again. I have been a member of the BIPP as well in the past - and I'm no longer a member of any of them.
Without going into detail about my own personal feelings about the organisation I'd say that if you are at the start of either a planned move into charging for your photography / going semi-pro or even jumping into a FT pro role from a previous job then it is worth joining. The convention is a great way to gather a lot of information, suppliers, and contacts - particularly if you are a wedding or portrait photographer. You still need to work out what is right for you - and dig underneath some of the self-promotion and the targeting of newbie pro/semi-pro photographers by some of the trainers who lurk on the forums/convention - although some of them *are* excellent....
If you are established, skilled, and set up with your suppliers then I can think of other organisations that you might receive more value from (and not only photographic societies).