What do you like about it?
URL dispatching: I was never a big fan of "/foo.php" is the file "foo.php" on my server, and if I want nicer URLs I need to mess around with mod_rewrite and keep that in line with what my logic in foo expects.
ORM: Because 90%+ of your queries, in my experience, do not need to be written by hand. Smart caching is much more important for performance, in general. You can always drop to raw SQL as needed.
Middleware and Signals: Easy to extend most parts of the request / response / view / render cycle without touching Django code itself.
What made you switch/use it?
It came out when I was disappointed with the Python web framework offerings. An easy sell for me.
How fast is it in production?
Hmm, to be honest, I've never cared too much. The webserver part of your app is (in my opinion) always the easiest to scale. As long as you use 'best practices' and share nothing, all you need to do is add a software load balancer and you can add new webservers until the cows come home. The first bottleneck people generally hit is database load - but Django gives you great caching APIs that help you alleviate that.
That said, I only skimmed this but it seems faster than Rails and the well known PHP frameworks: http://wiki.rubyonrails.org/rails/pages/Framework+Performance
How hard is it to master it?
I guess it depends how you define 'master'. The documentation is great, over the course of a decent sized site/app I think you'll use a little bit of everything, which is a great start.