The Kodak Zi8 captures video at HD 1080P, higher than the Flip’s quality of 720P. The Zi8 also has an external mic jack, while the Flip currently does not.
I’m planning to take a Zi8 out in the field for some tests in the near future.
Until then, check out this review of the camera from CNET.
If you already have a Flip and are wishing for higher quality audio, you might want to look into the Mikey for Flip recently announced by Blue Microphones. The Mikey is a small, square microphone that attaches to the bottom of your Flip and is designed to give you professional level sound.
The Mikey also has a mic jack if you want to use a different microphone. The device is set to come out next year. There’s just one eensy little drawback: the price. The Mikey is expected to sell for $70, about half the cost of the camera itself.