Sometimes. If a book has some really nice cover art I'm more likely to be drawn to it. However, as a fan of fantasy and sci fi I've grown pretty used to the idea that great books can have really bad covers. Some fantasy art is incredibly cheesey. I do judge ebooks pretty harshly if they've got Poser art on the cover (done with a program called Poser which creates oddly stiff looking human figures) because if they didn't care enough to pay a real artist I don't expect the story to be very good.
You can often tell the genre by just glancing at the cover since publishers use a sort of visual shorthand to do that. For example, true crime books usually have covers in mostly black, white and red. And then there's the coding for gender they do, where books by female authors are typically given a different look from books by men which can bias the reader before they even open the book. There was an interesting article about gender flipped coversin the Huffington Post recently.
http://www.huffingtonpost.com/2013/05/07/coverflip-maureen-johnson_n_3231935.html