Agent Jennifer Laughran of the Andrea Brown Literary Agency has a fascinating post on her blog about various stages of querying and the percentages of queries to requests to offers. See it here.
You might find the percentages discouraging but I don't, actually. If you get a personalized rejection, by Jennifer's accounting, you're in the top 15% of submitters. Not too shabby!
Yes, to only 1% of people who send her a full does she make an offer to rep them. But still - if she kind of likes something, but rejects it, there's a decent chance that another agent might like it even more.
The key, as always with subbing to agents or editors: Do. Your. Research. Check it out: 20% of the folks who sub to Jennifer get deleted without a look because they didn't follow guidelines. And it ain't hard to follow guidelines, people. If you can't follow simple guidelines in submitting your manuscript, how on earth are you going to survive when an editor gives you notes to change your manuscript? How can your judgement be trusted? Research, folks. After all the time and effort you put into your manuscript, a few minutes of agent research is more than worthwhile.
Been rejected and need some soothing? Check out her post On Rejection here.