I've been using the Chromium builds off and on for some time now and finally changed my default browser to Chromium. The two major reasons that sold me:
One. It is blazing. In comparison, Firefox 3.5 is now intolerably slow. I don't even want to try Firefox 3.0. This is both for rendering time and startup time. (Startup from a cold cache: 1 sec for Chromium, 10 sec for Firefox. That's right, Chromium loads in one second on my 8 year-old computer.)
Two. The "omnibox" (the unified area for selecting URLs, searches, and bookmarks). It works fantastically. Now, Firefox's implementation (the "awesomebar") is pretty admirable. They too support searching bookmarks and history and have a fairly sophisticated filtering language for narrowing the suggestions. But, as far as I can tell, in order to actually use any of the suggestions you have to move your hands away from the home row, down to the arrow keys to press "Down".
From a UI perspective, this is snatching defeat from the jaws of victory. (In Chromium, pressing Enter in the omnibox selects the first completion.)
Chromium is still a no-frills browser, and I still miss some Firefox extensions (okay, okay, only one). But it's already good enough that using Firefox regularly is just unbearable now. Hats off to the Chromium engineers!