I personally use fish, but mostly for the out of the box auto completions and highlighting, I don't use it for actual scripting, nor have it set as the login shell due to the 2nd reason you mentioned.
I mostly use python as well, and for some short commands I made pyxargs [1] which lets you run python code on stdin, or a walk of your current directory.
Things that didn't go well:
- Startup time is not great.
- Many, many things will assume that your login shell is either a superset of /bin/sh; or at least when given "foo", will search $PATH for /usr/bin/foo, etc.
- I ended up using Python features much less often than the regular shell commands.
That said, I would welcome a new "alternative" shell that can balance the expressiveness of Python with ordinary simplicity of plain old Bourne sh.