The boards are made from copper-clad kapton. I keep this around for rapid prototyping:

1) It's thin, so toner transfer is very fast! I can just stick it on a flat metal sheet, put that on a hotplate, and press down for a few seconds with a cloth.

2) I can cut it with scissors.

3) Most of my boards are single sided, and use SMD components. Although I can do double sided with this material. I 'plate' vias by just hammering a thumbtack through the board and bridging the two layers with solder. Or use a PCB drill bit if I'm feeling fancy. 

4) It's highly resistant to temperature and chemistry, and extremely low profile and lightweight. Sometimes this lets me do unexpected things.

5) in previous systems, it's thin enough that I can do direct printing by just taping it to a piece of paper. Presently I don't have that so it's indirect toner transfer and etch.

Apologies for the messy soldering job. I did use solder paste, but it was really old and dried out. So not as nice and clean as it could be.