I had some of the same concerns, coming from the Northeast. This really is a nice place to live, especially since it almost never gets below 50 in the winter daytime. The students here come from a wide range of places, so you wouldn't be the only new Californian.
As far as competitiveness (if that's even a word)? Doesn't exist. The 1L class right now is gaining a reputation as a bunch of gunners, but from what I know of the 2L class, everybody is pretty cool. No one talks about grades, since everyone knows that we're all a bunch of brainiacs/nerds anyway. Stanford has a reasonable curve (read grade inflation) and most of the professors clump, so there are few standouts. No one knows who is tops in the class (they have some sort of award for the top GPA, but I don't think they tell anyone who got it other than the winner). Cold calling (the socratic method) only exists in half the 1L classes, and even then it's pretty much painless as long as it's not a visiting professor (visiting professors go to town on you, but Stanford faculty are a bunch of fluffy pillows once they've been here for a few years). After the first year, cold calling is practically non-existant. In sum: this place is a breeze. Sun all day long, smiles all round.
Having said that, free time is a luxury. I spend more time on the golf course (twilight rate: 10 dollars for nine holes on one of the top 100 course in the country) than in the library, but apparantly that's not normal. A lot of people go on trips and do things in the city during the weekends, so leisure is not a total unknown. Just make sure to befriend cool people who don't study 24/7.
Stanford is also a great name to have on the resume when looking for jobs. The on-campus interview process for 2Ls is amazing, and pretty much everyone gets a job in the city of their choice, though it's not as glorious as a few years ago.
Finally, aspirations for law review if there's any interest there: it's not based on grades or a writing competition like a lot of schools. It's based on a rather gruelling editing exercise. In other words, you don't have to worry about it until May. For me, that was a major plus.