They look quite good on an application, not just for Oxbridge, for other unis too. Oxbridge will still rely a lot more on interviews, but there wouldn't be any harm in taking some. (Although a couple of colleges have made offers specifying distinctions in AEAs as part of the offer.)
I think they'd be quite interesting, if you enjoy the subject. My school doesn't enter people for them, but I've looked at some online, and kind of wish I'd made the effort to get myself entered for a bio one.
My mum looked at some online, and said that they're a lot more like how A levels used to be...