M1 it was easy, but boring
M2 it was easy, but Step 1 was annoying/sad times/waste of time
M3 started off fun because you are "finally learning medicine," but as the year goes on you get tired of the constant extra **** you have to do, constantly changing the way you present a patient to make someone else happy, constantly having to try to be "friends" with the interns and residents when many of them are very unhappy people, and having to constantly hear "you guys have it so easy these days, well back in my day" from the attendings as they pimp you over things that: 1) are guidelines and change from era to era, 2) have nothing to do with medicine or the patient, 3) are tiny details that no one cares about except their little branch of research. You just have to keep going, and have some fun along the way. M3 is the worst when it comes to having a life outside of medicine, but it is an intense time of learning and seeing medicine (much of it that you will never see again).
M4 not sure yet, I start in 2 months
My medical school experience has been a roller coaster - live for the highs, and hold on for the lows.