YouTube doesn't allow right-click and saving the file, no way to do that that I've found thus far. It temporarily spools the file to the browser cache, then the file auto-deletes when the page is closed. I've tried saving the cached file, doesn't work.
Anyway, I'm on a dial-up, and it does work, just takes long. If the playback is paused, the file will continue downloading/spooling, and will play smoothly when downloading is complete. As long as the page isn't closed, the clip can be replayed without having to be re-downloaded.
I've found that a download accelerator I have causes the clips to be cut short even though the download apparently has finished. If I disable the accelerator so the download can run at "normal" speed, it plays correctly.