I really enjoy going to the union - I mean, there's a great variety of debates and speaker evenings with all kinds of people you'd never think you'd actually get to meet in real life! (And some of them stick around in the bar afterwards, so you may get a chance to chat to them if you want!) You don't have to go absolutely every week to make it worthwhile, but most weeks there's something of interest, and if an event is popular, which it probably will be if you're particularly interested in going, you almost definitely won't get in unless you're a member! (In fact, for the really popular ones, even some members get turned away if they don't turn up early!) I also find it helps me a little to keep up-to-date with current affairs as once I'm in the 'Cambridge bubble' I start to forget the outside world exists - you know, that one besides lectures, supervisions, reading and essays! Good to keep a sense of perspective on the world and the much bigger issues out there! Plus I've got a good core group of friends that go to the union regularly, which is nice, and we often end up in big discussions of the debates afterwards!
Of course it depends on what sort of person you are, and whether realistically you are going to use your membership, but I reckon it's definitely worth it! Best money I spent yet in Cambridge probably!