You're asking the wrong question. No course is particularly "useful" for the MCAT, because the MCAT was never a content based test. You could learn the entire MCAT content in about 2 weeks if you spent eight hours a day. The real tough part is the critical thinking / problem-solving / speed and accuracy part. To train that, I think the best courses are some upper level math or physics/chem courses where you have to do complicated problems given a certain amount of information.
The reason I say this is because as long as you know the basic stuff for MCAT, you don't need much more. I didn't take Genetics or Microbio before taking my MCAT. There were 2 passages on Genetics (1 on histone subunits - I knew what histones were from bio 2, but had no idea that they were octamers), another on some ridiculous microbio stuff (trust me, i had never seen that before), and I still managed to pull out a 13 on that section. So, really what it comes down to is your ability to reason with limited information. Any course that trains your ability to do that, will be a useful course.
So that should be your #1 priority if you want to do well on MCAT. Recognize that knowledge will never separate a 30 from a 40, because they both know the same material, really. The guy with a 40 is just way faster. Now I do believe that courses like Microbio or Genetics would be marginally "helpful", I mean it couldn't have hurt if I knew about histone sub-units, but only you can decide what is a better use of your time: Learning about obscure stuff that is beyond MCAT scope, but may show up in a passage (not in the questions - the questions never ask you anything that is beyond MCAT scope)
OR
practicing and taking tests until your ass falls off.
disclaimer: You have to be conceptually VERY strong for MCAT. Every concept MCAT lists as within scope, you have to know like your own ass. I'm just arguing you get conceptually strong by doing questions - not reading the concept in a book.