It's worth less than a general studies grade, though some uni's will knock a grade down if you get an A in EPQ.
You have to put in a certain number of hours (it was something retarded like 200), and I felt like my time was better spent concentrating on my actual a-levels.
Plus, the actual essay is worth 10 marks out of 50, so it's not a test of your essay writing (despite that being the bit that interested me). The main marks are to do with your research. You also have to tediously log everything you do perfectly, join in with annoying feedback sessions with your mentor.
It's just preposterous, and not worth it. If you like writing essay's you're better off partaking in essay competitions. Cambridge do some good ones, but there are thousands.