there some to a be few open source finance project using Python out there (mostly for quote display) http://gliptak.150m.com/open_finance_projects.html
I have used it quite a bit recently for non-trading programming gigs. I like it, but it's got its limitations. It's a great quick-and-dirty language if you prefer OO.
We have used python on a couple of projects. It is a good OO-like language/platform. However , there are many good languages available which provide features similar to python. Python is in my opinion no better, and perhaps a bit worse than some of the existing products available. If the decision is up to us we tend to choose platforms/products which are tied to a large stable and existing company. The primary issue for us whether there will be support for the language in the future and the availability of talent to maintain solutions we develop: these last two points are question marks as far as python is concerned ..... Of course, being open source there is no upfront cost but this does not mean that total costs - including development maintenance costs over the lifetime of an application - are low.