I am really enjoying the series, but mostly for the stunning artwork; the opening animation is gorgeous art nouveau, and the series in general is well-animated and full of detailed backgrounds and distinct character designs.
Besides that, character interaction is one of the show's strengths. Victorique takes such a deadpan tone with Kujo and the Inspector (and everyone else), which makes for good laughs! Plus there is a lot of character depth that helps the viewer understand the characters' actions and personalities (eg. why Victorique is so stubborn and, at times, kind of mean).
The plot is interesting enough with relatively complex cases to solve, though recently the episodes have been more focused on the characters' back-stories and placed little emphasis on cases.
Overall, I highly recommend GOSICK to anyone who likes mystery and historical anime with well-rounded, complex characters. (Personally, though, even if the anime was lacking in plot or character depth, I'd still watch it just for the beautiful art!)