I think it depends a lot on the course. DS has just started at Cambridge doing a STEM course. It is very intense and fast-paced,and it's all condensed into short terms. He knew exactly what he was getting into though, he had researched so much into life at Cambridge that none of it has been a surprise and he seems happy there but does spend a lot of time either in the department or in his room working. He does love his subject, though, and is quite nerdy so is finding the challenge of the work fun and doesn't mind at all that he spends most evenings in his room working, he enjoys it. He was always a hard worker academically, and done supercurricular stuff for his own interest and fun. (Whereas, DS2 works hard at his A-levels but not AS hard, and has less subject enthusiasm, he prefers socialising with his mates, so I know he won't be thinking of Oxbridge and I don't think he'd suit it either/it would suit him.)
He definitely isn't a pubber or clubber but has given it a bit of a go recently but I don't think he is living your average student life, no. It is a lot of work. But the holidays are long! (expectation to do internships etc though, sometimes) But there are so many great opportunities and it's a really great city to be a student in. The quality of everything comes across. I was a student/have worked in lower tier universities myself, and there is a world of difference between them. Due to their age, Oxford and Cambridge are wealthy universities, and it shows. You only have to look at the departmental web pages to see how well organised it all is, compared to some other unis.
As someone else said, I think the admissions systems are extremely thorough. They WANT students to thrive there, and they know what they're looking for when they interview and look at their application and/or admissions test. I guessed myself that DS would be an ideal Oxbridge candidate so it wasn't a massive surprise when he was given an offer. I wouldn't have enjoyed the workload and high expectations, though, or the intensity, and I knew my younger son wouldn't either. He definitely wants a more "normal" experience.
What I would say, is that applying to Cambridge all came from DS. All of it. I was taken aback when he announced he wanted to go there, when he was in Year 12. Then he set about finding out exactly what he needed to do to get in, it all came from him. Every bit of it. He read and read about the process and continued to weigh it up as he went along whether he was ok with this or that. By the time it came to application time he was 100% sure, and I think that must come across to the admissions team.