My opinion is based on something not mentioned by others, namely that code at my work tend to live for decades (literaly), and hence that maintenance is very important to us. Maintenance of our own code, and the libraries we use. Our own code we control, but it is in our interest that the libraries we use, are maintained by others in the above mentioned decades or more.
To make a long story short, I have concluded that the best way to achieve this is by using open source implementations of Sun specifications all the way down to the raw JVM.
Of the open source implementations Apache Jakarta has proven to maintain their libraries, and recently Sun has done a lot of work in producing high quality implementations for Glassfish v3. In any case, we also have the source for all modules, so if all else fails, we can maintain them ourselves.
Sun specifications are usually very strict meaning that implementations conforming to the spec can be interchanged easily. Just have a look at servlet containers.
In this particular case, I would suggest having a look at JavaServer Faces simply because it is part of Java EE 6 meaning it will be available and maintained for a very, very long time. Then we have chosen to augment with MyFaces Tomahawk as it gives some useful additions, and it is a jakarta project.
There is nothing wrong with JBoss Seam or others. It is just that their focus is less towards the maintenance issue that is so important to us.
