I waste a lot of time between Vim and Python. I find it too slow to manually copy-paste from Python to Vim and vice versa. A good broken example is:
%
In any of your vim windows, type something like this:
for x in range(1,10):
print '-> %d' % x
Visually select both of those lines (V to start visual mode), and type the following:
:!python
Because you pressed ':' in visual mode, that will end up looking like:
:'<,'>!python
Hit enter and the selection is replaced by the output of the print statements. You could easily turn it into a mapping:
:vnoremap :!python