i took biochem (general) for one semester (didn't do the second semester of it) in undergrad. i think it's good preparation for MCAT, because it reviews and consolidates so much of what you learned in gen chem, intro bio, and (some) cell bio. it's also more complex; whereas you may have learned cell bio by memorizing things without understanding the mechanisms, in biochem you have to bridge the gap between chemistry and biology to understand the topic (which is the kind of thinking that can help a lot on the MCAT).
i would disagree with biochem being useless in med school. i know other med students who say that undergrad biochem was covered in the first week of medical biochem... maybe i got lucky, but for me, it gave about 4 weeks (out of 8) of a head start. that's not to say that you'll breeze through med biochem, but it's a lot of the same things; some students have never had to memorize the pathways and amino acids properties, so they struggle to remember all the minutia. though keep in mind that biochem is only one class, so if you can't fit it into your schedule, then don't worry about it (i.e.,nothing you do in undergrad can really prepare you for the shock of med school; you just learn to deal with everything eventually).
