Now a proud member of the Oxbridge Rejects group of parents I'd say give it a go but be very aware of the downsides if it goes wrong, which statistically it's likely to. It's all very well for @SpringRumba to make snide witty comments about the failures all being the next Mother Teresa but her child was successful, so she didn't have to deal with the fallout of a rejection and then the fact your child may later get detailed feedback (though you may also not get any at all) that confuses you as to why that rejection happened, as the process is so mysterious everything you do can look great on paper yet still not be deemed to be good enough.
You may think you and your dc will take a rejection in their stride and you're going to be cool and hands off throughout the process (I certainly did and made it very clear to dd from the offset that there were no guarantees, which she seemed to accept). The reality is it's extremely demoralising for a child to go through a long and torturous process like Oxbridge entry (the fact they now do the interview in your home makes it worse not least as we had tech issues on the day), which in our case involved tests, submitting written work, two interviews with the chosen college and then an extra one with the pooled one - and if you're pooled you can't help but have your hopes buoyed. Oxbridge entirely dominates the autumn term - you could go into it without any preparation but everyone on here is saying how much reading their dc is doing etc , how passionate they are about their subject, so it would seem unwise.
As a parent you can't help be invested in your child's disappointment and how they move on from there - the successful applicants have offers in the bag and can more serenely towards A levels, many unsuccessful ones are still waiting for offers from their second choices and have no idea where they'll be this time next year.
DD's from a school that only sends a handful to Oxbridge every year but this year that handful included her two bffs - looking at experiences on the Oxbridge rejects thread those kind of incidents aren't uncommon, I'd guess because Oxbridge applicants tend to flock together.
My take from it all is if my dd2 wants to apply (and I fear she will) I am going to strongly encourage her to do all her uni applications post A level with grades in hand, then to only go for it with straight A stars, and to go into A levels with a great gap year to look forward to, rather than having had a massive confidence knockback at a key point in their education. If in doubt I'd suggest people do the same - obviously gap years don't work for everyone though, you need to be sure you can fund them and your dc is doing a subject that would allow this.
Good luck everyone