Mike Cannon-Brookes' Is Tomcat Crap?, and the follow up Bashing the Cat are worth a look it you are a Tomcat user.

We are currently deploying to Tomcat, and we don't seem to be having a problem. But then, we haven't rolled out to any of our larger clients yet, so time will tell...

I'm not too concerned, because the problem seems mainly to be with Jasper, Tomcat's JSP component. Since we are using Tomcat mainly as a servlet container, this won't effect us. Besides, Tomcat 4.21 is out, with a totally new version of Jasper, so the problem may have been solved.

We are also using Tomcat to serve static content, and I think that we might need to look at this - an Apache/Tomcat combination might perform better here, and give us more fine grained control over caching.

Still, I'll keep Resin and Orion in mind if we have any scalability problems.

See also Using Tomcat in Production.