finished putting in the double pane windows.
there was a major snafu in that the measurements for the existing windows was complicated by two different dimensions of the existing windows (we measured the larger opening), so when the windows arrived, they were about 1/4" too big. so we sawzalled it away and installed them.
we also extensively caulked the outside, and the inside, and then placed the beauty strips to "hide" the replacement windows. we got single hungs, doubles were too expensive, prone to leaks. we didn't get sliders. we replaced aluminum single panes, as well as a double pane front window that leaked badly and also had those faux panes which obstructed view.
i started timing the time it took the heater to go off from different temps to our 57 degree setting, and it was about 7-10 minutes shorter. cycle time, ie the time it took for the heater to come back on was increased by about 20-30 minutes.
i took a couple of readings before and after installation.
no longer do you walk up to the window and it's freezing.
we found one significant draft after the windows were installed and two cold points.
the front door is not sealed well, and the two tubs are significant sources of cool as there is no insulation on the tub and they are both next to outside vents in the crawl space.
we use LP gas, not PG&E, so it's a little more expensive and less efficient, ie 15% less efficient i believe than natural gas. it makes our gas stove not as hot as well, so those steaks take a little longer to sear.
we replaced replaced 5 48x48 windows, one 8foot window (the front one), and two bathroom windows for $2000 for the windows at 25% off (my son works at milguard) and we installed them ourselves.
all in all, i'd say a decent investment. probably take 7-10 years to get our money back.
when we purchased the house, the previous owners had relocated the front door over a vent and left it unplugged in the crawl space. also, the venting is pretty poor, so some improvements could be made there as well.
a new thing out here is to use 2" pvc piping to pipe air throughout the house. it comes out the walls at about a 2-3' height. kinda neat, but obviously not easily retrofittable.