webapp, tipfy or django on google app engine
I would still prefer Django for its structure and a high support available over internet for it and for the following reasons: 

Webapp offcourse is light weight, but
Django comes with a nice structure
which saves a lots of time while
working on a large application.
Google app engine does provide a good
document for working with Webapp but Django has
a large community of programmers and
thus proves to be a better choice for
implementing some complex
applications.
Django provides a default admin
panel, which otherwise would need to
be created in Webapp, though Google
app provides an admin interface but
that is not equivalent of a full
fledged customizable admin panel.
Webapp itself follows Django for its templates.
which one are you using on google app engine?
what were the reasons behind your decision?