Not just me, but a lot of people will tell you that Seam is like no other application framework that has been created before. Don't confuse application framework with web framework (Struts, Wicket) or RIA framework (GWT, Flex). Seam pulls together so many technologies and gets them communicating. It's a collaboration framework in that sense. But it's more than what Spring does because Seam helps scope your objects and manages resources like the persistence context they way they were supposed to be managed. It also helps keeps things simply by using annotations rather than XML.
Is Seam perfect? I will be a zealot if I said that. Is it better than anything the Java EE world (or any other platform for that matter) has seen. Remember, Seam *needs* those other technologies, so it is not all by itself in the world. Seam is about making those other technologies look good.