I've sent out probably 200 queries over the last six or seven years. I got maybe, a half dozen hits. A few request for partials. I did actually land an agent once. He was a very nice guy, and loved my book, but he was in the wrong side of the business and it never went anywhere. Finally let him go. That was hard. Nothing on that one since.
However, I have since discovered that my queries a) sucked, and b) were rarely sent to the right people. And c) my books were too long. I've spent a lot of time over in the Query section of SYW and am now trying to figure out how to identify potential agents/editors. I'm also working on making my books more marketable.
I have found it helps if you are a) fabulously wealthy or b) famous. Honestly, what does Miley Cyrus have to say? Apparently if you are famous you don't even have to know how to spell your own name if there is a market for your brand.
Honestly, querying is the least efficient of all methods, unfortunately it is the only one open to most of us. It tends to take a long, long time, and often you will never get a reply at all. And when you do, 99% of the time (statistically calculated) it will be "I'm not sufficiently interested in your work to be the person to market it. This is not a reflection on yada, yada, yada..." Basically you have to find an agent who 'likes' what you've written. It can even be complete crap as long as they 'like' what you've written. Then they'll send it to an editor who 'likes' what they 'like.' Aristotle had something to say about this. Good luck.
-MM