The education:
My classes were almost invariably interesting and well taught. Of the 38 classes I took at Princeton, only one could be called anything close to poorly taught/designed (ORF 307 Optimization: *gag*). The combination of bright and engaging professors who were passionate about their work and a shopping period that allowed you to sit in on many courses before you fully committed to them made it easy to fill your schedule with only worthwhile classes.
[It wasn't until the semester after I took ORF 307 that I began to take full advantage of the shopping period. This feature is often ignored by the younger students who are used to a high school schedule largely set in stone.]
I found that not only were professors willing to entertain you at their mandatory office hours (the weekly time each professor sets aside to chat with students), they were also eager to chat with you after classes, to carry on lengthy email discussions about your latest questions and theories, and to schedule face-to-face meetings whenever the need arose. Most of the professors I've had were also hungry for student feedback: they care, and they'll gladly sit with you for an hour just to get advice on how to improve their courses.
This would be a good time to mention that Princeton really is an undergraduate-focused university. Unlike at most major universities, the Princeton graduate school is only a fraction of the size of the undergraduate body. This means that professors have more time to dedicate to undergraduates: both in terms of teaching and research opportunities.
Stress/happiness:
Stressful and overwhelming are apt terms for the Princeton academic load, particularly in freshman year when students are still adapting to the college environment, still figuring out that all important work/life balance, still learning to manage their time, still determining what a reasonable class load is, and just discovering that they're not the superstar in every single class any more (a first exposure to B's and C's is unsettling for many). Part of the problem is that the Princeton academic semester is shorter than most (there is some talk of changing this), and part is that Princeton classes cram A LOT of work into each semester.
I remember feeling particularly stressed in my first semester when I took Physics 105 (an advanced mechanics course), Math 215 (single variable real analysis), Spanish 207 (studies in Spanish language and style), my Writing Seminar (the Race Debate in the Modern U.S.), and Computer Science 126 (General Computer Science), because I felt like the stream of work was never ending. Whenever I finished one problem set/paper/homework assignment, it was just in time to start the next, and there was seldom time to explore a subject more deeply than what was called for by the class assignments. I realized by the end that I had overloaded myself. Even though these were the classes that I placed into when AP/SAT II scores were considered, there was no reason why I had to take five of them, and no reason why I had to take them all at once. (Side note: as a Bachelor of Arts, you will never need to take 5 classes in a semester, but Bachelors of Science and Engineering need to take 5 courses roughly every other semester.) Additionally, most people would recommend taking fewer courses or an easier load when taking your writing seminar, since this class can demand a lot of your time.
In any case, I survived, as most people do, and in the subsequent semester I thought more carefully about the balance of my classes: how much time would I need to spend in class/lab/precept, how many problem set versus homework versus paper-writing classes should I have, how large and difficult would the workload be for each class, how many midterms and finals would their be and how much of the grade depends on them, did I leave space for lunch? Although it's tempting to just jump into all of the classes that sound interesting to you, I've found that this sort of holistic schedule planning can do wonders for your subsequent semester stress level (and I have to say that my second semester was much less stressful!)
To sum up:
Is there a lot of (dare I say, too much) work? Yes!
Can it be stressful? Yes!
Can it be mediated? Yes!
Student motivation:
The beautiful thing about Princeton is that in nearly every field, you'll find someone who will amaze you with his/her knowledge, accomplishments, or passion. The most common question during freshman year is, "Why did I ever think I was good at X?" (The trick is to channel that shock and awe into inspiration. :) ) You have to remember that Princeton is home to some of the most accomplished and passionate young adults in the nation; I never failed to be amazed by anyone I met there.
Intellectual atmosphere:
I have to admit -- the prospect of having regular, spontaneous intellectual discussions with other students was one of the aspects of college I most eagerly awaited. This did happen, but not to the degree I had hoped (in part because I wasn't actively initiating these discussions and in part because of busy schedules). Princeton did offer many ways to inject a less spontaneous but by no means less satisfying level of peer discussion into my regular schedule though. These included weekly discussion clubs like Paideia (a dinner-discussion club, led by a different professor each week) and Sustained Dialogue (a student dialogue and action group which focused largely on topics of race at Princeton). These club meetings often sparked longer discussions/debates among students that would last late into the night. They also offered good ways to meet other students interested in intellectual discourse.