I have done several showcases, though none recently. I prefer the more targeted ones (wedding, jewelry, etc...) to the overall showcase, which is cheaper ($7 vs. $15). I always get a lot of views, but as far as I know only one sale from it.
But like I said, I haven't done one in a long time, and I agree with other posters that I'd rather try out advertising on FB or someone's blog to see how those compare.