I really like Ladonna.  I think they've managed to build her into a good character in a very short period of time.  In just one episode, you really get a good idea of her virtues, flaws, quirks, emotions, etc.  Her loud and slightly obnoxious Southern Gal nature should serve as a good source of fun in the future.

As we've seen with George and Fern, seemingly insignificant/new characters can have a way of becoming more popular than even central characters if they're developed properly.  I anticipate -and hope- that this is the case with Ladonna.  It seem to be the direction the producers are trying to take her.

By contrast, new characters who just sort of fall by the wayside and get forgotten have been generally uninteresting, or lacked traits to distinguish them from the main cast.  Mei-Lin or the Molinas, for instance.  They lack any real dynamic.  Nothing about them is particularly sympathetic, amusing, endearing, etc.  They're just there as tokens to represent other culture, it seems.  The only good thing that came out of them was a "grody" soccer ball...