Scribus? Desktop publishing? Huh?
Yes, indeed :) Scribus is a tool to create all sort of publications from newspapers to two-fold brochures for your company's booth at an exhibition to a fashion magazine. You create a layout, you fill it with content, you export it to PDF and send it to print bureau. The job is done.



Will a print bureau accept PDF files I created in Scribus and print them as I expect?
If you didn't make any significant mistakes — definitely yes. Please note that Scribus feature a preflight verifying tool that checks your document for most typical errors before creating a PDF or PS file. And before you save the PDF file make sure you've chosen printer intended output at the Colors tab. This will convert all colors to CMYK. Scribus is known to create high quality PDF files that match expected standards.

Is it anywhere close to Adobe's InDesign or Quark's XPress?
It very much depends on what you do. Certain things are harder to achieve in Scribus, but one thing you can be sure if: Scribus always creates high quality no-problem PDF files.

How do I do imposition in Scribus?
You don't :) There was an attempt to create an imposition tool in Scribus and it failed. Serious imposition tools are quite complex, so creating a simplistic solution, while better standalone tools already exist, seems to not having made sense to Scribus developers. For list of solutions please visit this wiki page.