It's incredibly time consuming, expensive, and mentally challenging.
The first two years, you take about 1 college semester worth of classes/labs each month, including several 3 hour "final" exams. You are graded on a curve, so a certain percentage of students fail every exam. There are extracurricular activities, clubs, and usually some service opportunities, but clinical opportunities are pretty limited. Many students spend up to 12 hours a day on schoolwork - even weekends.
The third and fourth year, you rotate through several clinical settings - one specialty per month. During those rotations, you will be graded on your professionalism, interactions with patients, and understanding of the clinical application of medicine in that specialty, as well as a final exam that you will study for with very little formal teaching. You may spend 60 hours each week in the hospital and still need to find time to study for exams.
Of course, this is all before residency - 80 hours in the hospital each week, while getting paid less than you would have if you'd just gotten a BSN, and you still have to study for 2 major tests - USMLE Step 3 and the Boards (each are at least twice as long as the MCAT, and much more detailed and difficult).