I wanted to typeset a 6x6 grid, with each cell framed, and a PDF image in each cell -- and with the center 2x2 square replaced with another image. Except for the center "window", it is trivial with pdfpages. With the window, I could see no good way to eradicate the frame lines in those cells, or put them back after the fact.

I also tried doing it in four blocks (above the window, to the left/right of the window, below the window). The main problem there was the scaling. I saw no easy way to scale the 2x2 left/right blocks the same as the 6x2 top/bottom blocks, since the scale factor was not predetermined and not known. (I think I'll ask the pdfpages author to write out the calculated numbers in the .log.)

I ended up using plain \halign, just the way the TeXbook says to do ruled tables :). The entire template was &#&\vrule width.5pt #\cr (the leading & repeats it as often as needed). Turned out to be way easier to have complete control in this case.