Query letters can be such a challenge to write. Especially the paragraph where you're trying to sum up a 400-page book in a brief and interesting way that will excite the reader, make her want to request the book.
I usually write the query letter as early in the novel-writing process as possible---when I've got a pretty firm idea of where the plot is headed. That way, as I write the novel, I can continuously go back to the query letter and tweak, tweak, tweak. Set it aside, read it again. Set it aside, read it again.
That little synopsis/summary paragraph is VITAL. In fact, it (and not the author bio or the cute little rhetorical-question opening of the letter) is probably the most important paragraph in a query. I believe it's what "sells" your book idea. In fact, it really should feel like the summary on a book jacket cover. It should contain detail (not too much, not to little), mystery, and a "hook." And, it should flow effortlessly, leaving the reader wanting more.
So, it's worth all the time and energy to get it right. And usually, you'll know when you do. When you can read it and not stumble over the words. When you can picture yourself in a bookstore, picking up this particular book, and reading the jacket cover and thinking, "Hey - this is something I'd want to read."
Honestly, putting yourself in the reader's shoes is probably the best way to know if you've tweaked it enough to get it right. But it sure does take a lot of time to get there. ;-)
**edit - well, how's this for good timing? I just read some great query advice on the Behler blog (she's a publisher): link here