Well, I have seen the open source version of MySQL be used for many high volume sites without problems. That said, Information as to the differences between MySQL Community Server and Enterprise Server can be found at the following URLs:
http://dev.mysql.com/downloads/
http://www.mysql.com/products/enterprise/features.html
http://www.planetmysql.org/kaj/?p=64 (very informative)
I have not yet used the enterprise version of MySQL. Apparently the Enterprise project just split from the open source project last month, but it has good reviews. It looks like they basically trimmed down the community version to only allow functionality that is very solid and tested while the community edition allows for some experimental functionality. I would say that using Enterprise edition is a good idea, and the included support doesnt hurt.