I'm taking a dental class and the test was so easy you didn't even need your notes but so many people received low grades. My thing is if you are not serious about learning why do you spend your money to come to school an failed. Oh and by the way I got a 100 on my test!!!
Open book tests can be tricky though. Usually they're timed and if you don't know the material already, wasting time thumbing through a textbook is NOT going to help you. It usually hurts you. Teachers know this.