I am an overseas member living in Ireland and joined the SAA earlier this year. For me the benefit is having access to a wider range of art supplies than I can get here and having them delivered free if over £50 helps.
The downside is that as overseas members we don't get free postage on everthing nor do we get insurance for exhibitions. We also miss out on access to exhibitions etc without having to fly of course. And we pay extra!
All in all though I think it is worth it although it would be nice to see more for overseas members.