Tomcat vs. Other App Servers

[Curtis Spencer]

Hi, I have been working with Tomcat for a few months now with little problems.  I am wondering if this app server is ready for prime time when it comes to business end of the things.  What do the other servers that cost upwards of $10000 offer that Tomcat doesn't?  Also, I have been hearing a lot about Caucho's Resin Servlet engine as another open source alternative?  Anyone have any takes on the app server market?   Thanks Curtis