It was the best thing I did at school. Even if universities don't pay attention to it, hell will it help with your course (if you are doing an essay-based course). I had people the end of my first year unable to reference still... But the most useful thing is that you have had practice at how to write and organise a large piece of written work. Most people's first attempt at this is a piece of university work that counts for a large part of a module. Everyone has a lot of experience with short essays, but not with long ones.
Finally.. it gives you a chance to write something on your own interests, rather than following a course. Can't complain at that. I didn't get great A levels, so if I ever have to talk about that area of time, I'll try and deflect interest away from A levels and focus on my extended project.