As far as the courses and such, the first year is broken down into 3 units:
Unit 1 being meant as a PBL introductory unit which covers a broad set of organ systems
Unit 2: cardiovascular, respiratory, and renal
Unit 3: gastrointestinal, hematology/oncology.
Second year consists of 2 units:
Unit 4: musculoskeletal, neuro, behavioral science
Unit 5: reproductive and the life cycle
Between Unit 5 and the start of 3rd year you have roughly 9-10 weeks to study for the USMLE Step 1, which was nice. Our students have done pretty well with the boards, so if you are under the impression that PBL students perform poorly on national exams, you are mistaken. In my class of 63 or so, our average score was higher than the national average, and 15 people scored above 240, with our top scorer hitting a 268. Actually, Blade28's class probably did pretty well also because their match list was freakin awesome.
As far as difficulty is concerned, because unit 1 is an introductory unit, it is actually pretty chill (except for those few who look at the broad range of topics covered in unit 1 and end up trying to learn everything right away and become overwhelmed), and unit 2 is where the workload picks up and such.
Testing is based on a set of exams at the end of each unit, as there are no midterms or quizzes in between. To pass you need to pass those set of tests covering gross anatomy, pathology, clinical skills, and the PBL cases. This is an oversimplification of how things are tested by the way, as each unit differs a little from the other.
Traditionally, roughly 4-5 people out of a class of 62 fail the unit 2 exam, so it's not to say that the courses are easy. Actually, medicine in general is not difficult per say, it's just that the volume of information you are responsible for is HUGE.
Because of the PBL curriculum, I had tons of free time to study on my own. I would say on average I was in the classroom 5 hours a day, which gave me tons of opportunities for my beloved mid-day naps.
Judging from the experiences of my friends in other medical schools, it seems the 3rd and 4th years aren't too different throughout the nation, as it's really the first 2 that differ.