My first reaction to Flock was "holy crap!" - after using it for awhile, I'm still very impressed, but also a little surprised. I'd thought that the tighter integration to my blog, del.icio.us and Flickr accounts would have totally changed the way I worked with those tools. I suppose there is still the potential for that, but there isn't really anything that I couldn't replicate in other browsers using some decent bookmarklets. My del.icio.us account is already one click away on any browser I use (for adding, querying and viewing), as is my blog (again, for all CRUD tasks), Flickr, and lots of other handy tools. And my current blog posting utility (using the popup window in any browser) doesn't give me the crappy-HTML-code-generation problems, or disappearing-post problems that Flock's built in editor appears to suffer from.
After stepping back a bit, I realized that part of what was impressing me so much about Flock was the Firefox core beneath the custom code. The extensions I'd added are pretty sweet (GreaseMonkey, the Web Developer Toolbar, etc...) and already available for Firefox. And "stock" Firefox doesn't have some of the perceived performance problems I'm having with Flock (some of which are, I'm sure, due to the synchronization with del.icio.us).
So, after saying I'd give Flock some time to try it out, I waffle on that, and am moving straight to Firefox for a week or so. I'll be keeping my eye on Flock though - it's got a lot of promise, and will only be getting better as the code gets massaged.