On my system (Window 7) undo/redo seems to mess up after a bit, which was quite annoying. It'd eventually start removing lines or something. I don't remember exactly, but in the end that's why I stopped using it. Also, I think I remember long running scripts freezing the IDE.

NotePad++ does a decent job for editing a file here or there. PyDev is my editor of choice when working on a project.