Twisted

What do you like best?

It's free, fast, asynchronous, and handles concurrency like a dream. Even more important, it hides a lot of the low-level networking code and lets me build advanced apps quickly. Need to listen to a TCP binary stream? Easy, and you don't need to mess around with sockets or anything directly. It's web server component is also very fast and handles multiple connections well. Great for building out web services, UI's, and anything network related.

What do you dislike?

It's a big framework, and it can be a little intimidating at first. Once you understand the basic concepts and event loops, it should make more sense.

Recommendations to others considering the product:

If you are into python web dev, give it a try