They're either bitter or bland, but my, they're pretty.
Long ago I read that you can use cornflower (or knapweed) flowers like blessed thistle (Cnicus benedictus). Well, yes, most of the Centaurea flowers I've tasted have been bitter, and they can therefore be used, like blessed thistle, as simple bitters.
There's not a lot else to them.
And the usual garden beauty, bachelor's buttons (Centaurea cyanus), is not bitter at all. Instead, it tastes of raw wheat kernels. And while not medicinal, it's the only one of the lot I use. It makes my herbal tea blends prettier.
The flowers are fairly easy to pick, and if you dry them too hot (like 60-90 deg. C.) the flowers will pop right off on their own.
Nice plants, knapweeds.