I had a fantastic intro genetics class before med school, and it's been really helpful.
Our MS1 biochem course first semester was basically mol bio, which was poorly taught and assumed all kinds of prior knowledge. I understood a lot of material only because of that previous class (e.g. RFLPs, microarrays, most of the current techniques). Also, they assume in many classes that we can easily and fluently read pedigrees, and again the previous class really helped. We will not have MS1 genetics until March.
I'd say that if you can get a good instructor who teaches modern genetics with plenty of mol bio, it's a good investment. As others have said, the USMLE increasingly emphasizes these topics. It's hard to get a good instructor anywhere in med school, and there isn't much time to learn it.