My Quantum sails (#1, main) have windows, and I'm very happy with
them. Saved me from smacking into someone on the opposite tack last
weekend (I'd been hit by the boom and was a little dazed).
Two sail panels are simply replaced with the reinforced clear
plastic. You can see a picture of them at: www.sfsail.com/E27sm.jpg.
If you need a better picture, let me know and I'll see if I can find
one.
The windows are just in the right place, in my opinion. There's also
a small window high on the main to see the trim of the jib or genoa.
This is kind of cool, but usually pretty hard to see through.
--brendan