I have not taken the MCAT yet but I did start studying for the bio section before this semester began and it was tough. Now that I have finished this semester (physiology, anatomy, genetics) things that were tough in the BS were being brought up countless times in each of those classes. I feel I am 10X more prepared for the BS section after taking those classes than if I didn't have them. I am a human bio major so I had to take them, but I didn't they were too dreadfull, with enough dedication you should be able to walk out with a good grade and well prepared for those topics.
I've spoken with students already in medical school at my university and after hearing which classes I have taken and what class I still intend to take (biochem) before the MCAT, they all agreed that I am more prepared than I can imagine.
Personally, physiology is the most important recommended class, I believe. My Kaplan review book stressed each system seperately which would have been a ton easier had I completed physio earlier.
Hope that helps.