I was a student member when I was studying in the UK and became a graduate member. The conferences where good and free, the library was good for some project I did.
Must admit I have membership lapse, as outside of UK nobody asked me for it and only thing that got me up the management ladder is hard work and grabbing opportunities. Actually you get to a stage where they do not look at you BEng(Aero), your reputation gets you the positions (or is that because I am in the maintenance field)
I suppose membership might help if you want to work for one of the old UK coporates and also as mentioned if you are a consultant