I went to a couple of them last year, they are good for revision, byt do not expect to be taught anyhting new. More so the lectures they had for the sciences were worth it but the ones they had for 2U English were not that impressive, most of the suff said was covered greatly in school.
I would go if you wanna quick revision of what you have done as well if you want to reinforce what have you learnt. But going to these lectures will not help you learn new topics or vastly improve your understanding in other topics. You need good old fashioned studying for that IMO.