I have both a Flip and a Zi8, and the Zi8 beats it hands down in almost every way. Picture quality in particular is much better, the fact that it takes SD cards, doesn’t try to use some silly proprietary software on the computer, has external mic support, replaceable battery, high speed mode (60fps in US models), and I don’t see that it’s any more complex to use than the Flip makes it my first choice of the two. You just turn it on and shoot, unless you want to get more complex.
The one flaw: the rounded bottom on the Zi8 means you must have a tripod or some other base for the Zi8 to stand and shoot. The Fip’s flatter bottom is easier to balance on its own.
Also, the video from Zi8 isn’t directly compatible with the iPad camera connection kit. The videos load and can be transferred off, but cannot be previewed on the iPad.
I’d never tried the Flip with the iPad before this morning (Didn’t have the camera connection kit in Taiwan) but they are directly compatible with the iPad CCK, so that is one point in favor of the Flip.
Kodak does, finally, I believe, have a newer replacement model for the Zi8 now.