Nathan Bransford has a great blog post about bragging. And the comments yield an interesting discussion as well.

Bransford’s point is that you should “brag” on yourself all you want in a query letter. Share your relevant achievements and accomplishments. However, then turn off the spotlights. “When it comes to describing the actual work in a query, we really don’t want to be told how great it is,” the agent writes.

So list your relevant credentials and don’t be shy. But don’t describe your book as being life-changing, the best book ever published, or amazing. And don’t say your work is as good as Hamlet