Literary agent Nathan Bransford has done a series of posts allowing readers to understand the query system and how difficult it is to sift through piles of queries to find the potential bestsellers.
It starts here, where he randomly picked 5 queries out of 150 entries readers send in.
Then you go to day two, where he's posted the first 30 pages of the novels. Based on which query you liked the most, how do you like the manuscript?
And finally the wrap-up.
It's harder to do than I'd anticipated. Following his guidelines, to pick the query that I felt had good writing and a story I thought would appeal, I picked one query. The manuscript pages for that didn't live up to the interesting premise of the query, and I ended up picking a different manuscript from the sample pages.
It shows the importance of a good query letter though. It doesn't matter how good the novel is if you can't get an agent/editor is interested.