There have been been very lengthy discussions on this forum and elsewhere regarding Seam adoption in the Java space (esp. corporate), Spring vs. Seam (esp. AOP vs. interceptors and static injection vs. dynamic injection), and what parts of Seam (e.g. performance) can be improved in Seam 3.
But suffice it to say that if you look at JSR 299 (CDI), you will get an understanding of what's to come in Seam 3 core. The Spring component model and
platformis a mature one that materialized prior to the release of EJB 3 in EE 5. Seam and EE 5 were released too late and now how many Spring-based projects in production are going to integrate with or refactor to Seam?
Seam and the rest of the EE 5 stack has a very high learning curve but you'll appreciate the stack like I do (esp. after having gone back to a J2EE 1.3 with Struts 1.1, etc. project!) once you have worked with it for 6-12 months.
And there definitely are some very large companies using Seam, but it's not as popular as it should be...