I'm certainly interested in giving it a go, so I can compare it with Coda.
I've been comparing it to Coda, and so far, Coda wins.
I don't like the way Espresso manages projects and I don't like the way the live preview works, which are two of the things I love about Coda.
It isn't bad though, not at all. I don't think it's an app for the serious developer though, unless they just do front-end work. I haven't had enough time with it yet, but I didn't find any way of setting options for a testing server, and so previewing of server-side scripts seemed, well... impossible. This could be something I've missed though, so I won't say such a feature doesn't exist.
Espresso is certainly good in terms of looks though. The interface is as pretty as hell, the syntax highlighting and default font is great (although this can be changed in Coda, I haven't had the time to "lift" Espresso's for use there yet), the code navigator is really very good. Perfect, probably. Espresso seems to handle tag completion a little better, and although rarely used, code folding could be useful.
It could be – and in most cases I'm sure it is – that I'm just not used to Espresso yet after being spoiled by Coda, and also that Espresso is a young app, but I couldn't use it full-time (and as I don't do app-switching, couldn't use it at all), but if you don't own Coda, it's certainly worth buying and using.
I'll be giving it a full trial this weekend to see just what it can and can't do, and make my decision then, but I think I know what it will be.