Apologies that this is a cross post with a comment in James's recent "New Features in Illustrator" blog post, but I thought it was worth repeating here.
I’ve just realised that the new shape builder tool also works on lines – as well as closed shapes. This means that you can take a bunch of overlapping lines and trim them in less time that it's taken to type this line. : )
Apologies if I'm late to the party and the rest of you know this, but trimming lines (as opposed to closed shapes) was a major pain in previous versions – now it’s just a simple click and drag!