I have taken an anti-Query Shark stance. I believe Janet Reid's work is great in getting an overblown query down to size, but that 250-word limit is not a set standard. (Use it only when sending to the Shark herself or if an agent asks for the 250-word limit ... I've seen exactly one other who requires that.)
My greatest advice on queries? Tailor them to the agent you are targeting. Check out the agent's web site, blog, etc. to discover what his or her main points are, then point out how your novel fits into his or her view. There are still pitfalls. I wrote to one agent who liked my work but rejected it on one point ... it wasn't the correct narrative voice being sought for that agent's representation. Who could have guessed?
I am not sending out more queries now. I am waiting for replies from several I sent out recently, and those may take two to eight weeks. In the meantime, I am going to get back to writing ... and it's a different project that the one I have already started. The already-started project is linked to my first novel, and I'm going to hold off until I see what the response is to that first novel. I have another novel in mind, in a completely different genre with completely different goals.
I just need to get back to putting fingertips to keyboard. I need that creative fix ... now.