No need to copy files to the iPad, Plex streaming works great even over 3G. You can register your server with myPlex then sign on Plex clients to make getting to your video collection super easy.
Custom video categories are fantastic for keeping track of programming screencasts. I just setup my Plex with Peepcode, Destroy All Software, and Railscasts categories and am very pleased with the setup. If I start watching a video on the Roku box on my TV, I can pick up where I left off on the iPad or my iPhone whether I'm at home or not.