Sight glass is useful, but does have one major drawback. When you are boiling it bubbles and boils in the sight glass too, making it impossible to tell actual volume during the boil. It is very useful when you are measuring water for the mashing process, making sure you draw enough during sparge and post boil.
Sight glasses come in both glass and plastic, and plastic ones come in both hard and flexible. I have found that just rinsing/flushing water through it works. Be sure that you have an easy way to remove the sight glass, then you just run a brush through it if something gets stuck in there.
Cheers!