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