We bought our current home 9 years ago and it was built in 1961. The home had the original Lennox furnaces and we decided that rather than wait for them to fail, we'd replace them at our leisure. At the time, natural gas cost quite a bit more so we figured that it would be more economical over the long run to have new mid-efficiency furnaces. Due to the layout of our basement we couldn't install high efficiency furnaces. We've been extremely pleased with their performance and have them maintained each year by the Furnace Company.
We highly recommend that if you spend the money on new furnaces that you spend the $140 or so dollars and protect your investment. The Furnace Company has a deal where if you pay this year for your next year's maintenance you get a discount which we take advantage of.
The installation had its challenges because of the age of our home but Derrick's guys handled everything that was thrown at them. They were very clean, polite and professional. They were also young which initially concerned us but it was clear once they started the job that they were more skilled than many tradespeople much older than them.