In terms of total number of users and questions, we're second only to the Gaming and Web Applications proposals - and when you consider that those sites have a much greater community intersection with Stack Overflow, it's pretty damn good.
We have almost as many questions as either of the other two, a higher answer rate than both, and more views/day than one of them.
Subjectively speaking, the site seems to have a lot of activity and the activity seems to be going up. There are no questions older than 24 hours on the front page anymore.
But best of all, we have actual professionals and serious enthusiasts that are actively participating. For a fledgling site that's Stack Exchange's first attempt to branch out to non-computer fields, I'd say that's pretty encouraging!
Of course, it's not my call, and no decision will be made until the end of the public beta, but if that's not what you'd call success, then I don't know what is.