I just re-queried an agent who'd requested the full manuscript a couple of years ago, and who'd asked me to keep her in mind if I did any rewriting. Yesterday, the good old US Postal Service brought this: We greatly enjoyed the first 50 pages of your manuscript and are hoping to read more. Please send the full manuscript.
And guess what! The rest of the book is much stronger than the first fifty pages. I think. I don't even mind the two rejections I also received yesterday.
Off to the post office!