I gave this a bit of a try yesterday and parts of it I'm really impressed with and others I'm disappointed in.
It looks great (OK.. that's a huge understatement.. It's beautiful!), the syntax highlighting is very nice and it feels smoother to use to me than Textmate in some ways. (Not being stuck with proportional fonts like in Textmate is a huge bonus to me, those fonts give me a major headache after a long session.)
Unless I'm missing something there's a few features I'm not seeing.
1) Preview through a local test server.. I have a feeling I just missed this, I could paste the local.site/page into preview and it worked but couldn't get it to take that info in the sites preferences.
2) Dragging and dropping an image from a finder window into Coda's text editor does nicely add the full tag including height and width but even when working in a site with a html 4.01 strict doctype it adds the xhtml closing bracket. (Did the same with an image in a .css file as well, so it's not context sensitive at all) If I could find that option, and to set it to write it as root relative (ie. /folder/image.ext) I'd be thrilled!
3) Where's the option to not connect to the sites ftp on startup? Most of the time I'm working on the test server and / or a brand new site. I'd rather it not connect until I am ready to upload. (Makes me nervous that I could accidentally send up a bad edit and not notice :S ) Honestly I'd prefer to disable ftp entirely in it and use Transmit when I'm ready to upload.
I'll probably play around with it a bit more today, so I'm sure this will be solid as well, but I think Textmate is closer to what I'm looking for in an editor.