see here: viewtopic.php?f=2&t=3344
After having been thru the first winter now with some pretty cold temps (for the area) I would have to say that these windows are probably a little more effective at keeping the cold out than keeping the heat in. Again, they make a noticeable difference but not nite and day. They did stop the sweating inside also which was nice.
But after making the calculations, in my case, I will say that it will take many years for these windows to pay for themselves at present energy costs. Of course if rates go up (pretty much a certainty) the payoff will come sooner. Every case will be different and there are many variables which will affect a specific homeowners outcome. Windows are just one part of the equation.
Also , in my case, we open our windows alot! - and one of the nicest things now is the ease with which they operate - pretty much 2 fingers to slide them open. The double hung operation also makes it nice to be able to open the top half instead of the bottom because of stuff sitting on the window sills.