Honestly, I took the NBME 6 4 days before my exam and scored well (>250) and I thought my actual exam was much, much harder. I guess it depends on the form that you get on exam day, but I couldn't believe how hard it was. I think NBME 6 paled in comparison, but I don't have my score back so who knows. I would say just be prepared to go in and read fast (very long stems) and be mentally tough knowing that the questions may be very difficult.