I teach EPQ in years 12 & 13. Both my DDs did it. In terms of the skills it covers, it is a great stepping-stone from GCSE/A level spoon-feeding to university study. We do the WJEC version as the AQA one was too inconsistent.
The skills covered include planning, research, synthesising information, analysing sources, evaluating and presenting your findings in addition to the dissertation or artefact. My own DDs found they smoothed the transition to university; DD1's 3rd year literature review was like a longer and more in-depth version of the EPQ.
For students who are unsure what they want to study, it can really help them to make up their minds. Someone studying Bio/Chem/Psy for A levels could be considering medicine or psychology. So why not look into something along those lines? I have a student in y13 whose EP is engineering-based but who has now applied for medicine. He was concerned about this being brought up at interview, but my advice was to say that he chose the topic at the start of year 12. Whilst he has enjoyed it, it has made him realise that Engineering is not for him as he would miss the day to day interaction with people which is key to medicine.
One of my y13s is aiming for PPE. She has written & illustrated a children's book in a foreign language (nothing to do with Politics and she is not studying the language for A level). She has already had an offer from Cardiff stating that if she gets a grade A/A in EPQ, her offer is reduced across all 3 A level grades. Unless her final presentation is dire, she is heading for an A. She is hoping to present to me the 2nd week back.
As far as completing it in year 12 is concerned, yes it's a good idea to get the bulk of the reading/research done then plus a draft of the final outcome. However, both my DDs finished in year 13. DD1 put most of hers together in the Christmas holidays, DD2 had written her essay before that, but spent Christmas sorting out referencing & source analysis. Both got A* or A and both had cleared it before their mocks.