I admit that cover art is huge for me, as is a title that makes me curious. If one is bad and the other good, that's okay. I just need one to catch my attention. From there I read the blurb on the back. Then I open the book. I read the first paragraph, and I can usually tell by the end of it if it's a "Yes" or a "No" for me. If I'm not sure, I'll go to paragraph two. At the end of that: Did the book suck me in? Did I try to keep reading on beyond those first few paragraphs, with my 4 yr old tugging on my leg and shoppers trying to push past me to get through the aisle? If so, it's a definite buy. And if it's from certain favorite authors, I'll buy it regardless. They've proven to me, over and over, that they hook me in no matter what it is. If I end up with a stinker from them I can deal with it. They usually don't let me down.
If it's a series, and I've enjoyed the first book, I'll buy the next. Even if the reviews are horrible. Just to form my own opinion. The exception to the series rule for me is "The Hunger Games." I didn't read it until recently, and after some of what I read about "Mockingjay," I'm wary to move on to book two. The first one was so good, I don't want to end things with disappointment. So we'll see--the internal jury is still out of that one.