My house was built back in '71 and had the old single pane, double hung wooden windows. In the winter you could feel the cold coming in, the propane furnace worked overtime trying to keep a comfortable temp but ti was still drafty and there were cool/cold spots in every room. In the summer the A/C struggled to keep a tolerable temp often running nearly constantly during daylight hours when outside temps hit the high 80's/low 90's and even then barely managing to keep the inside temp below 80 by late afternoon on the really hot days.
We had replacement windows installed by N Ga Windows and Doors late last fall. We went with their 'house window', the exact brand and model escapes me at the moment but they're low E, double pane, argon filled and have a life of the structure warranty. There was an immediate decrease in noise from the outside and the perceived comfort level improved dramatically. According to the stats from my thermostat, summer run times on the A/C have decreased by 30-40% for the same period last year. And the A/C is actually cycling on and off throughout the day, even on the hottest days, rather than running constantly like it did prior to the new windows. I don't have stats for winter since I didn't have the fancy thermostat installed long enough but seat of the pants comparison of propane usage show a savings of 20-30% for 2016 from the prior one. Granted, there are some variances due to the weather year to year but overall I have to say it's one of the best investments/upgrades I've done to the old place.