I got 4 by Peter's generator... and I've only got one more year left at Oxford (unless I end up staying for clinical school). And I'd have to concur with the conclusion that it's absolutely not been worth the money for me. I'm a medic, so I rarely find the time to queue for hours to see a celebrity speaker, and I've only been to 2 debates because they tend to end quite late and, because I'm at Hilda's, getting home safely is an issue. Being a geek, I don't fit into the social scene at all, so not only have I never stood for election, but I'm also not close friends with any hacks (although I did like poor Sabina very much; such a shame, what happened to her), so I've never been invited to Pressie's or any other event with free booze. My college library has a good collection of fiction, and my departmental library is sex on legs, so I've never needed to use the library other than twice for cheap DVD rental. I went to one ball, because I had a pretty dress I wanted an excuse to wear more than once, but it was, as Jen said above, held in a conservatory and when the free wine ran out, the only option was to use the cash bar, which charged me about £7 for a double spirit and mixer. And I wouldn't even have gone to that ball if I'd not had a friend who was going to be queueing for four hours for tickets anyway and didn't mind getting me a ticket while she was there. I tried to use the Union bar a couple of times in my first year just for the sake of recouping some of the membership fee, but always felt awkward and out-of-place amongst the rahs and socialites.
I might get more value for money in my third year, because my boyfriend (who's ex-public school, surprise surprise) likes the Union and goes there fairly often. Which I suppose serves to illustrate the point that it's very difficult to work out, before you go to Oxford, whether joining the Union represents value for money. You can't be sure what kind of friends you'll make, or whether your JCR will have a free DVD library, or whether you'll have friends who want to queue with you for four hours to get ball tickets or see a famous speaker. To a large extent you won't even be able to determine that after the first two weeks (in which the membership fee is discounted). So IMO it's pretty hard to predict whether the Union is worth joining or not. It's something best worked out retrospectively. Personally, I'd say, if the membership fee isn't a big deal, then join, but if you'd have to struggle and make sacrifices to find the money, then don't bother.
Edited to say: thanks for the anon rep, whoever you are <3