Code organization is basically up to you. The default organization is the typical MVC directories, but that's not a requirement for anything (and you can add other directories / subdirectories as you wish).

The one caveat on code organization is that Play! hot-compiles all code under its /app directory. My approach to sharing code with other non-Play! projects is to use "ant" to build a jar, and add that to the classpath of my Play! project. The end result is that you get hot-compilation of everything specific to your webapp, but you have to use ant to build code outside of that. I find it works fine in practice, and if it's a standalone app then you never have to leave hot-compile land.

Maintainability is great -- Play! makes heavy use of ThreadLocals, so you can access the current Request, Response, template arguments, etc etc no matter where you are, without having to pass it all around as method parameters. So making helper methods is super easy. We created a "helpers" package to put functionality shared across Controllers and can follow DRY without a problem.

One more question - you mentioned the Groovy templates are slow, do you mean as in execution time? Have you run into any performance problems?

Also, the documentation is absolutely first rate. I think this really counts for a lot. Compare the Play documentation with, say, Struts2. Chalk and cheese.