I have seen the window and have a sample in my office. It is well built and the windjamber is a nice option. when looking at a window, always take note of how it closes...is there a lot of movement in the sashes when its locked? if so, air will find its way in. Also how much weather stripping is on it.
Please let me know if you have any further questions
Actually jayal11, compression and closing a window have nothing to do with air tightness.
I have surveyed many a window that have a very tight compression seal that leak copious amounts of air when clamped shut.