I know this might not suit you, since you made it clear that you want a nifty GUI program, but I've been using this
one-line command in Windows, as part of a batch script that does Full Text Searches in Calibre Library and converts the results snippets to pdf:
Code:
ebook-convert file.epub file.pdf --unit millimeter --font-size-mapping=10 --custom-size 155x280 --pdf-page-margin-left 15 --pdf-page-margin-right 15 --pdf-page-margin-top 15 --pdf-page-margin-bottom 26 --pdf-standard-font sans --pdf-page-numbers --page-breaks-before "/" --chapter "/" --change-justification justify --pdf-mark-links --authors "Calibre FTS" --book-producer "Calibre FTS"
In the actual script I have even added a custom CSS, using the option --extra-css, and it does the job quite rapidly...