INFINITELINE
This is a promising start and I'm this close to buying it, but it's just missing a feature or two that would make it a no-brainer purchase.
Like other comments, the interface is slick and pleasant to use. Also, you can edit directly from your remote source, this allows for proper display of pages using php, server-side code, etc. The included reference books and validation work well and are polished. Coda also seems pretty stable; it only crashed on me once during a few days of work.
As far as the missing... The current level of integration that we see in the interface is not carried over to the underworkings. One example of this is that the styles of html files with linked style sheets are not recognized when you switch to the css editor. Also, there is no shortcut to apply any styles within the code editor (you should be able to select text and click on a style to format it). This highlights the current anemia of features that are needed when writing code from scratch. The auto-complete and blockedit functions are nice. Nonetheless, more options to avoid typing (perhaps similar to TextMate's bundles) seem necessary to bring Coda up to speed.
To summarize my list of needed features:
Overall, a good start from the Panic team. I will likely buy Coda in the future, especially if the above features are implemented. Future releases will likely make Coda an indispensable and useful tool for web development.
(Version 1.0)