Will spread sideways to form a small patch if happy, moisture seems to be a requirement for continued success.

Thanks, bboy,
Should I take this to mean it didn't take over your whole perennial bed? I have heard stories of some campanulas strangling the roots of perennials & shrubs with their own roots, and coming up several feet away from underground runners. Any trouble like that?
Thanks.
Laura

You can read what a Puget Sound gardener says about it on her very excellent website.

Here is a link that might be useful: Korean bellflower

Thanks, hemnancy!
I appreciate the help from both of you. I still haven't moved it, too busy. I have to figure out what space I'm willing to let it run wild in, or else move fragile stuff away from it just in case. It sounds less badly-behaved than some, and it sounds like how it performs really depends on light & water (which is true of all plants, I suppose!)
I saw a white one (similar campanula, I don't know which one) in a garden the other day and it had gone all over the place among the other plants.
Hmm.