Actually had him for LS2; Arnold moves at a rapidly FAST clip. However, everything's contained in the lecture slides so study those well, and note the particular example he emphasizes in class. Those usually end up on the short free responses on exams. A rather average lecturer; he goes over the slides and explains them, but I didn't feel he did more/explained it better than the book did. If you can keep up with his pace and memorize everything, you'll be fine. Got an A in LS2.
Arnold was a but confusing at times but tried his best to clear up misunderstandings. His exams were extremely difficult and tricky. I often relied on my TA for help because he was so terribly confusing. Overall though Arnold was not horrible. You have to pay close attention in order to do well.. and seek outside help. lots of it.
Arnold taught the biology aspect of GE Cluster but did not lecture as much as the other professors. His studies involved bird song studies and other animal research which he related back to human biology. He basically reads straight off of the slides but is open to further explaining his points.