I've been trying a few solutions for good video playback on the E51.
With h.264 encoded video, Realplayer does not play back at all as it indicated it was not supported. Realplayer also did not support chapters or rotation.
I've been using Coreplayer and it did not like 320z240 H.264 video at 700kbps. It had terrible playback with motion still most of the time. But it could do rotation of videos to use the most of the portrait style E51 screen.
With re encoded videos using MPEG4 instead of H.264 at 320x240, video playback is great in Coreplayer with no dropped frames. One thing good about Coreplayer compared to any other portable video solution is its support for chapters. You can encode the video with chapters and you will be able to see this in Coreplayer and jump straight to it. Even with iPods, while you can jump to the next chapter, you can't see what chapters are available and can't jump straight there.
Can share what are your tips for optimal playback of video?