For the record, all I know is that Halo 3 is a video game that everyone loves.  Anywho…

The short answer to that question is YES, practice tests are great. But the real question that should have been asked is “what practice questions do I want”?  First, you want to get old exams from YOUR professor.  Different organic chemistry professors focus in on different things.  Your professor might focus in on physical organic chemistry (orbitals and such) where as another prof might focus on synthesis.

Next, if you have the time, will, and extra food, surf the net to find other profs at other schools and their tests.  If you can answer both sets (yours and the other guys) of questions, you are well on your way to a nice fat A.