These are HUGELY important. I can't emphasis that enough. Especially when often the first a reader sees of a book is a thumbnail of the cover art on a mobile device. The cover indicates the genre, and the title can too. You need to hook your reader with a combination of these two, and entice them into finding out more. That's when the blurb kicks in. It has to be absolutely perfect. Worth spending hours, or even days, over. Test it out on your friends. Make sure it's spot on, and will make the reader want to buy the book. They must feel that they simply HAVE to find out what happens in the story.
I'm pretty happy with how they all turned out. I have spare chapters I could add into "Vanquishing the Viscount", if people start favouring longer books again, but currently the market seems to prefer the shorter ones.