The ease in which it can be setup with very little configuration required! It has been able streamline our development process by allowing us more time spend coding rather than configuring servers/applications etc.. We have not found any downsides yet as we are still using this software but I am sure there will come along soon enough if you use Apache Tomcat or other similar products that do offer some sorta support like mod_jk does (which was one reason why i chose tomcats). If your looking at deploying Java EE apps then look no further because they really make deployment seamless compared others out here such as glassfish OR JBoss AS7+TomCat+.