First, a shameless plug: I write queries, lots of them, and I do it to help clients I ghostwrite books for as well as other writers. If you need help with yours or your book proposal, send an email or give a call and let's talk (for free).
Not that that is done: the answer is I don't know and you probably don't either. The one fact that we know coming into this business is that we will receive a ton of rejection. Our family and friends will wonder what the hell is wrong with us that we would take up writing. Then we look for someone, a mentor, who can give us a leg up, but they could care less and tell us to go pound sand. And then we write essays and short stories to see what happens and they get rejected over and over, but maybe a few get accepted and so we feel confident enough to write a book.
This is when the rejection really begins. Many agents simply won't respond to your queries nor will some publishers. Many publishers won't even peak at your query because you aren't an agent. And if they do, many will say sorry, but no. Many will say it isn't special enough. And others will tell you you don't know what the hell you are doing and their advice is to go hire a consultant or quit.
Then there are the reviews, if you do get published.
So with all of that rejection, it's natural to wonder if the query letter we write for our precious manuscript sucks or not. No agent or publisher will tell you so you are left to reason that if it is rejected every time (so far) then it must suck. The problem is, it may not be bad. You just haven't put it in front of the right eyes yet.
It is important to be flexible and listen for the signal amid the noise for changes you may need to make, but it is also important to stick to your guns. Constantly changing the query thinking the next fix will make it work isn't a good plan either. Ergo why it's important to have other writers and editors read your query for their thoughts and to know enough to disregard much of what they say and keep the gems.
Overall, if you have done your homework, studied sample queries, developed a unique voice and message for your book, and created a succinct appeal, then have some faith. It only takes one yes and when that happens, you will know the query worked.