We use IBM WAS Developer Edition in our development machines for free.
There is an Eclipse Plugin provided by IBM, so no need to buy IBM Rational Application Developer or Rational Software Architect tools.
Starting with version 8, WAS provides a special folder called monitor deployment. Once you put the .war or .ear file in there, it is deployed automatically without human intervention. This greatly helps us in our continuous integration server. Once the deployment binary is ready, we write a script to copy it to that folder and then, voila! The application is up and running and accessible from its context root.
IBM needs to pay attention to market changes more quickly. We now have Java 9 and very soon Java EE8. We do not want to wait for two or three years after their release until they are supported by the new version, say WAS 10 in our case.
Installing or configuring a WAS server instance as a Windows Service causes a lot of problems, especially when the server needs credentials to stop.
More than five years.
No stability issues at all.
No scalability issues at all.
I used Oracle GlassFish and Apache Tomcat but found them good only for small applications. Also, they need a lot of XML configuration and are not really production-ready.
The initial setup is straightforward and, thanks to Java and its platform independency, it requires almost the same steps in all operating systems.
You just have to install IBM Installation Manager, define your repositories, and click Install or Update.
If your application is just a web app that does not need to scale big, you can obtain a single core license of WAS Express edition, which has almost the same features with limited processing cores.
If you manage a very big application farm (i.e. need to run 10 or more WAS servers) it is better to get IBM WAS Hypervisor Edition.
I evaluated Red Hat JBoss EAP, but since we need to integrate with IBM MQ and the IBM Integration Bus, all in a mixed Windows/AIX environment, we found that IBM WAS was the winner.
IBM products, in general, need a lot of experience. They are very similar, have much in common, but to avoid getting into trouble and strange issues, it's important to read within the IBM Knowledge Center before starting. Also, do not skip any step in the installation/migration/configuration section, as it may stop you later, even without your knowing it.