I am using cameras right now since my rooms aren't attached in this semi converted house. They work great for just basic hand communication and such and to see what is going on when the headphones are down and such.
However there is about a half second of latency which makes them worthless for queuing, watching fingers, or any other sort of live visual queues since they will arrive late.
The windows and sight lines for iso booths and people in the control room are great since you get the realtime feedback. Basically a lot of the same feel of playing live together, but sonically separated. But in reality it still takes a bit of getting used to since you feel like you are in a cage.
So for guys that need that I have them play with headphones in the room.
Soundproof windows are a bit expensive, but not really that much in the grand scheme of studio construction. All depends on your workflow of course. I track more in the control room than anywhere else.
Also consider you will need two cameras so they can see you as well, which is also important. Add monitors and TV's, video extensions, etc. the cost gets up there. I use a laptop and webcams (higher latency, but cheaper). With webcams you can only have one per USB bus. So I have to use the laptop cam.