What is most valuable?
Tomcat Application Serve is an open source Application Server.
Compared to other application servers like Jetty, Weblogic etc. this is very light weight
Tomcat can be very easily configured to suit your needs.
A large number of ever growing web community is there to help you out, in case you face any issue.
Tomcat has been known to be very stable and has been deployed and used by many well known organizations.
Also, it does not require too much of memory(RAM) to start with. It can start running in a very low memory and uses memory on as needed basis.
What needs improvement?
There are certain well known compatibility issues with some implementations of JDK
Tomcat doesn't give you a performance like Apache when you have static HTML contents. And it is more suitable for hosting servlets and works best as a servlet container.
Scalability is a major problem with Tomcat. When your application grows out of proportion and you have applications that draws heavy traffic, then Tomcat is not the right option and you might have to look for other options.
What other advice do I have?
Tomcat is one of the best application servers and servlet container. It is very light weight and application startup consumes very low memory and it uses memory on as needed basis. Can be run on various platforms like Windows, Linux etc. Not suitable for hosting static HTML content and Scalability is a major problem with Tomcat.
Disclosure: I am a real user, and this review is based on my own experience and opinions.
Some other advantages of Tomcat are: It is easier to embed Tomcat in applications (e.g. in JBoss), it implements the Servlet 3.0., JSP 2.2. and JSP-EL 2.2. support and it is easy integrated with other applications such as Spring.