Cahokia is worth a stop for anyone visiting St. Louis. It is a nice side trip from the city. If you are visiting the area solely for the Cahokia Mounds you may be dissapointed. The site is deserving of its place on the world heritage list, the site IS important. However, as a tourist site Cahokia does not offer much besides mounds that are evidence of a unique civilization.
The climb up the stairs to the top of the mound gives little knowledge of the site without being paired with the video in the visitor center.
The museum should not be overlooked, it is an excellent source of information about the site, and the video provides a insightful view of what makes Cahokia special. Overall, I enjoyed the experience as I am very interested in Native American history. For me 1.5 hrs was sufficient to enjoy the museum and site itself. On a clear day from the mound top you can see St. Louis in the distance.