"Rails is much more traditional and familiar to most programmers. Java programmers with years of experience in J2EE can switch to Rails quickly and all they have to learn is what drudgery not to do any more.
Seaside, on the other hand, is an entirely different paradigm. It's new, fresh thinking. And for certain narrow domains it can definitely offer significant improvements over the traditional request/response model.
But I don't see it as something that's going to catch a ton of mainstream traction the way Rails has. It's simply too different for too many people. And for the "most applications most of the time" sphere, it doesn't offer enough benefits over something like Rails to be worth the mental rewrite for most people.
I certainly encourage you to take a look regardless. It'll broaden your mind."
I would not encourage you to take a look since it is true what Andy Bower once said:
"Smalltalk is dangerous. It is a drug. My advice to you would be don't try it; it could ruin your life. Once you take the time to learn it (to REALLY learn it) you will see that there is nothing out there (yet) to touch it. Of course, like all drugs, how dangerous it is depends on your character. It may be that once you've got to this stage you'll find it difficult (if not impossible) to "go back" to other languages and, if you are forced to, you might become an embittered character constantly muttering ascerbic comments under your breath. Who knows, you may even have to quit the software industry altogether because nothing else lives up to your new expectations."