Impressed w/my current Armstrong straight A/C (1993) Rock solid, even though the heat exchanger burners were screwed up by installer-really haven't had any issues.
Thinking about adding Lennox to the mix next spring (along w/Amana, AS, Rheem,York, & Bryant) when getting estimates.
Old Lennox HVAC seem (from what I know) to be extremely durable. My Father in law's is 1985 HP that's still going strong w/zero maintenance, impressive. I've talked to others about how tough their old Lennox HVACs are.
Can newer Lennox units match their older counterpart's reliability and durability. I've read somethings of newer units that lead me to believe that its a big no