[Koha] Comparing book cover services

In Koha, book jackets can come from Amazon, Google Books, Syndetics, Baker & Taylor, LibraryThing or Babeltheque. In my production opac (not Koha) I'm using covers from openlibrary.org. I'd like to know experiences, opinions, comparisons, ... about these services. Thanks to everybody. Stefano -- Dott. Stefano Bargioni Biblioteca della Pontificia Universita' della Santa Croce - Roma "Si apud bibliothecam hortulum habes, nihil deerit" (Cicerone)