Engine Optics Pipes
 

Just come across these: http://www.engineoptics.com/index.html

On the one hand they seem like a good idea to keep an eye on your coolant on the other hand I think i'd be constantly worried that I had a weak point in my cooling system.

Anybody have any experiences of them?

That's quite unusual.

Will it not over time leave scale like inside and look scummy?

You'd think so wouldn't you.