I really can't compare Princeton to the other Ivies, since I've only been to Princeton.
You can have a great work/life balance at Princeton; you just have to plan, and maybe get a little lucky. Personally, last semester I managed ~8 hours of sleep per night, plenty of time for friends, a campus job, and five classes. (Most people take 4 or 5 per semester.) But you have to pick your classes wisely -- if all five of your classes are assigning weekly homework due on Wednesday, that's going to be a problem.
FightTheTide already hit many of the aspects that can make Princeton's academic schedule difficult. For me, the toughest parts academically were 1) learning to study and 2) taking time-consuming, hard courses in subjects that I was bad at. Number two is mostly because I'm an engineer -- I really sucked at intro physics and it dominated my freshman year. As a sophomore I learned to create more balanced schedules filled with challenging courses AND easier courses that I actually liked.
I don't think that Princeton is probably all that more difficult than its peer schools. Maybe a little more difficult than some, but...meh. It depends very much on what classes you're taking, how much you're doing for independent work, etc.
Also, I have NEVER experienced negative competition amongst students here. All competition that I've experienced has been of the constructive, I-want-to-do-well-for-myself sort of thing, and even in classes whose grades were totally curve-based (physics, math -- gotta beat half the class to get a B+, or whatever) people form study groups and help each other out all the time.
