It's a production-ready web container, that does exactly what it is supposed to do. It's fast and lightweight, suitable for a nice development environment.
If your application does not need the extra features of a full application server, Tomcat is a great choice.
Very flexible (yet somewhat complex) security framework. If your application uses Spring Framework and has security constraints, this framework is a must.
It has a steep learning curve, but once you understand its main concepts its quite easy to integrate with almost any user repository.