I have spent the weekend writing a GUI in Python. It's not too bad, but not that great either.
Using Python is quite easy and I'm sure I couldn't have written the same application in C as fast. The automatic memory management helps a lot too.
Fast as the development is, it is also quite painful for someone (me) used to static syntax and type checking. The ten edit-test cycles needed to fix a stupid error (defining a variable that shadows a built-in function) certainly were not faster than ten edit-compile cycles in C.
I'm also a bit worried about shipping an application without a way to make sure there are no "obvious" typos in the notoriously hard-to-test error-checking paths. Adrian probably knows what I mean.
And yes, I know about pychecker. It helps somewhat, but it executes all code outside of class definitions and class __init__ methods, which breaks horribly if the code fails a check and calls sys.exit.