The results usually aren't too bad...back-cover copy is pretty formulaic, and you can't go too far wrong just typing in the copy from a published book in your genre, and replacing the key words.
The key thing is that you're trying to sell your book, right? So you say things that you think will make people want to buy it. That's good thinking, but sometimes I wonder if the authors have actually read their own books, the cover-copy is so far off the mark. Comparisons are made to books without the faintest resemblance either in plot or voice to the book in question. Allusions to authors, styles, and literature movements that the author hasn't been exposed to since 11th grade Lit class 30 years ago are demurely smuggled in. Characters are identified as the main character who aren't the main character. Really.
I understand the need to talk up your book, but really* - don't compare yourself to Crime and Punishment if the only similarity is that a crime occurs in your book and the perpetrator suffers because of it. It's like comparing your book to the Bible because it has God in it. And don't say your book draws from the surrealist movement just because you threw in a couple of vaguely weird bits that nobody would have noticed if you hadn't made a character say "Wow, this is so weird, I feel like my watch ought to start melting soon."
Moral: Back cover copy should both sell and remotely resemble the book you have written. It's bloody hard, but I promise it can be done.
*titles and genres have been changed to protect the innocent