My general rule of thumb is

if customer.dictates('specific language')
use('specific language') # usually Java / PLSQL / .Net
else
use('Python')
except error('too slow')
use('C')

I love the simplicity and power of Python and will use it to prototype proof of concepts (not so much GUI work - would use HTML or .NET for that). For me, Python really makes programming a lot of fun again and though it is slower, I haven't yet had the need to shell out to C code though I expect I will soon.