I'm coming to Espresso from the TextWrangler + CSSEdit user perspective.
Plusses: The live HTML editor feels smooth and responsive, the code completion works relatively well, code collapsing is nice, and the editor overall feels quite a bit like TextWrangler (which is a huge complement).
On the CSS front, almost all the wonderful, intuitive smoothness of CSSEdit is there and working smoothly--live preview, code completion, and the property editor frame is beefed up and even easier to use than before. Lots of CSS3 goodness in there as well.
And, the app so far has been entirely stable for me.
Minuses: The HTML editor has no one-click validator. Seriously? How is that not a #1 priority feature? Having to copy and paste (or direct upload) my HTML to the W3C validator manually from an all-in-one web development app is kind of ridiculous.
The CSS editor also has no validator, which is even more ridiculous since CSSEdit had one before. Removing an incredible time saver (and frankly workflow necessity) from the replacement app is just unforgivable, and it's almost enough to drive me back to CSSEdit, and has kept me from paying for Espresso, as much as I want to.
Also, the CSS editor has ditched CSSEdit's full color picker for a silly, under featured pop-over dialogue. I can see how the color picker could get confusing switching from one color thing to another, but the lack of a color wheel is a total deal breaker--if you're using the editor to test-drive color schemes, the inability to eyeball complementary colors from a wheel is an absolute deal-breaker, and makes the color selection system borderline useless--I just end up going to another app to pick my color and paste it in, which is ridiculous.
I guess you could keep an image of a color wheel in another app open and use the magnifying glass to grab colors from it, but that's silly.
My only other complaint is that as a full-site workflow, the project system doesn't "feel" right to me--it works well for a hierarchy, but doesn't really seem to do anything for managing a large project or keeping track of complex site structure if you're using Espresso for anything other than template creation or a very simple site of a dozen pages or so. Basically, it doesn't scale well.
Bottom line: So close to being a great app, but removing the color picker and one-click validator that were already in CSSEdit is a total deal breaker, and the lack of a validator in the HTML editor in version 2.0 is ridiculous. Fix that, and I'm buying an upgrade immediately.