I have used web2py for the last few years and have been happy with it:

- well designed components: DAL, SQLFORM, views, etc

- commitment to backwards compatibility

- works on GAE

- helpful community (groups.google.com/group/web2py)

- thorough documentation (web2py.com/book)

The main criticism of web2py is that its design is not 'pythonic', which is irrelevant to me.