I built a couple of web.py apps and here are my observations:
-- Advantages --
* Minimal configuration (No configuration file, just run a single python file in the simplest case)
* Getting started is really fast
* import whatever libraries you like (PyMongo, SqlAlchemy, Mako, Beaker, etc) - No lock in
* The Web.py source is straightforward enough that you can go directly there to figure stuff out
* WSGI compatible
* No Admin interface and extra scaffolding
* Very simple URL routing interface
* Simple cookbook is available for adding basic features
-- Disadvantages --
* Smaller community
* Not a lot of documentation or guides
* Some problems will require you to look at the source
* No built in user management/auth
* Web.py development community isn't adding new features rapidly
--Conclusion--
Probably good for rapidly building simple web apps. It seems like the support/infrastructure needed for a large-scale production app isn't available. However if you are willing to build out your components it will give you maximum flexibility for a larger app