I owned the CD400 and loved it! But I wanted the higher quality MPEG movie feature of the CD500, so I sold the CD400 and bought the CD500.
The 500 is a great camera too! However, I could no longer tolerate the EXCESSIVE startup time and in-between picture time of the 500. It was MUCH slower than the 400. Not to mention that you have to hold the camera almost completely still while its writing to the CD. It's kinda of annoying to ask family and friends, who wanted to see the picture immediately, to gather around and wait for it to come in. Plus I got tired of explaning to everybody not to jostle the camera while its writing to the CD. If you jostle the camera too much, it gives you disk access errors.
After 6 months I sold the 500. I plan to wait untill May to check out the DSC-W1 and the DSC-P100. They look pretty sweet.