I've been using iShowU to create a series of OS X video tutorials (screencasts) for my mother. I'm impressed! It's one of only 2 or 3 OS X apps that can do this, and it's only $20. Works fine and you can edit in iMovie. I'm going to experiment with exporting from iMovie as a "iPod" video, which in my past experience is surprisingly viewable.
For screencasting use create a very simple user account and run the screen res at 800x600 (or 1024x768). Screen rate is 2 screens/sec and set size to match the res (800x600). Stop and start with the shortcut keys. I'm experimenting with output.
For viewing the choices seem limited. I wonder if .Mac would work better! I'm currently using a link, which in Firefox causes the .mov file to be downloaded then played by Quicktime. I'm also staging via Google Video, but then in it plays in very low res Flash (like all shared video I can find).
What I want is a "free" (ad supported) video sharing site that would stream using the better Quicktime codec.
Update 1/1/10: I'm impressed! This works extremely well. I do my screen captures now using H.264 for compression and res 800x600 with "thousands of colors". I set the low refresh rate to 1 fps and the high refresh rate to 4 fps and choose the 'use low when mouse not moving' option. A short tutorial screencast takes only about 6ooK. I upload the videos to my Google web creator page as files and then create a blog post pointing to them. I don't use the video pages because the files are so small I don't need to stream, and the QT H.264 codec gives far better results than Flash video.
Update 8/17/10: iShowU is still around, now with 10.6 versions and a "pro" version. Price is very reasonable, the upgrade price is just the delta between the original and new price. Unfortunately, the've adopted an insane licensing/copy protection strategy. You need to consider this new hassle in your purchase decisions.